Peeking Iterator
<p>Design an iterator that supports the <code>peek</code> operation on an existing iterator in addition to the <code>hasNext</code> and the <code>next</code> operations.</p>

<p>Implement the <code>PeekingIterator</code> class:</p>

<ul>
	<li><code>PeekingIterator(Iterator&lt;int&gt; nums)</code> Initializes the object with the given integer iterator <code>iterator</code>.</li>
	<li><code>int next()</code> Returns the next element in the array and moves the pointer to the next element.</li>
	<li><code>boolean hasNext()</code> Returns <code>true</code> if there are still elements in the array.</li>
	<li><code>int peek()</code> Returns the next element in the array <strong>without</strong> moving the pointer.</li>
</ul>

<p><strong>Note:</strong> Each language may have a different implementation of the constructor and <code>Iterator</code>, but they all support the <code>int next()</code> and <code>boolean hasNext()</code> functions.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input</strong>
[&quot;PeekingIterator&quot;, &quot;next&quot;, &quot;peek&quot;, &quot;next&quot;, &quot;next&quot;, &quot;hasNext&quot;]
[[[1, 2, 3]], [], [], [], [], []]
<strong>Output</strong>
[null, 1, 2, 2, 3, false]

<strong>Explanation</strong>
PeekingIterator peekingIterator = new PeekingIterator([1, 2, 3]); // [<u><strong>1</strong></u>,2,3]
peekingIterator.next();    // return 1, the pointer moves to the next element [1,<u><strong>2</strong></u>,3].
peekingIterator.peek();    // return 2, the pointer does not move [1,<u><strong>2</strong></u>,3].
peekingIterator.next();    // return 2, the pointer moves to the next element [1,2,<u><strong>3</strong></u>]
peekingIterator.next();    // return 3, the pointer moves to the next element [1,2,3]
peekingIterator.hasNext(); // return False
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 1000</code></li>
	<li><code>1 &lt;= nums[i] &lt;= 1000</code></li>
	<li>All the calls to <code>next</code> and <code>peek</code> are valid.</li>
	<li>At most <code>1000</code> calls will be made to <code>next</code>, <code>hasNext</code>, and <code>peek</code>.</li>
</ul>

<p>&nbsp;</p>
<strong>Follow up:</strong> How would you extend your design to be generic and work with all types, not just integer?
['binary-search-tree-iterator', 'flatten-2d-vector', 'zigzag-iterator']
[]
[{"value": "cpp", "text": "C++", "defaultCode": "/*\n * Below is the interface for Iterator, which is already defined for you.\n * **DO NOT** modify the interface for Iterator.\n *\n *  class Iterator {\n *\t\tstruct Data;\n * \t\tData* data;\n *  public:\n *\t\tIterator(const vector<int>& nums);\n * \t\tIterator(const Iterator& iter);\n *\n * \t\t// Returns the next element in the iteration.\n *\t\tint next();\n *\n *\t\t// Returns true if the iteration has more elements.\n *\t\tbool hasNext() const;\n *\t};\n */\n\nclass PeekingIterator : public Iterator {\npublic:\n\tPeekingIterator(const vector<int>& nums) : Iterator(nums) {\n\t    // Initialize any member here.\n\t    // **DO NOT** save a copy of nums and manipulate it directly.\n\t    // You should only use the Iterator interface methods.\n\t    \n\t}\n\t\n    // Returns the next element in the iteration without advancing the iterator.\n\tint peek() {\n        \n\t}\n\t\n\t// hasNext() and next() should behave the same as in the Iterator interface.\n\t// Override them if needed.\n\tint next() {\n\t    \n\t}\n\t\n\tbool hasNext() const {\n\t    \n\t}\n};"}, {"value": "java", "text": "Java", "defaultCode": "// Java Iterator interface reference:\n// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html\n\nclass PeekingIterator implements Iterator<Integer> {\n\tpublic PeekingIterator(Iterator<Integer> iterator) {\n\t    // initialize any member here.\n\t    \n\t}\n\t\n    // Returns the next element in the iteration without advancing the iterator.\n\tpublic Integer peek() {\n        \n\t}\n\t\n\t// hasNext() and next() should behave the same as in the Iterator interface.\n\t// Override them if needed.\n\t@Override\n\tpublic Integer next() {\n\t    \n\t}\n\t\n\t@Override\n\tpublic boolean hasNext() {\n\t    \n\t}\n}"}, {"value": "python", "text": "Python", "defaultCode": "# Below is the interface for Iterator, which is already defined for you.\n#\n# class Iterator(object):\n#     def __init__(self, nums):\n#         \"\"\"\n#         Initializes an iterator object to the beginning of a list.\n#         :type nums: List[int]\n#         \"\"\"\n#\n#     def hasNext(self):\n#         \"\"\"\n#         Returns true if the iteration has more elements.\n#         :rtype: bool\n#         \"\"\"\n#\n#     def next(self):\n#         \"\"\"\n#         Returns the next element in the iteration.\n#         :rtype: int\n#         \"\"\"\n\nclass PeekingIterator(object):\n    def __init__(self, iterator):\n        \"\"\"\n        Initialize your data structure here.\n        :type iterator: Iterator\n        \"\"\"\n        \n\n    def peek(self):\n        \"\"\"\n        Returns the next element in the iteration without advancing the iterator.\n        :rtype: int\n        \"\"\"\n        \n\n    def next(self):\n        \"\"\"\n        :rtype: int\n        \"\"\"\n        \n\n    def hasNext(self):\n        \"\"\"\n        :rtype: bool\n        \"\"\"\n        \n\n# Your PeekingIterator object will be instantiated and called as such:\n# iter = PeekingIterator(Iterator(nums))\n# while iter.hasNext():\n#     val = iter.peek()   # Get the next element but not advance the iterator.\n#     iter.next()         # Should return the same value as [val]."}, {"value": "python3", "text": "Python3", "defaultCode": "# Below is the interface for Iterator, which is already defined for you.\n#\n# class Iterator:\n#     def __init__(self, nums):\n#         \"\"\"\n#         Initializes an iterator object to the beginning of a list.\n#         :type nums: List[int]\n#         \"\"\"\n#\n#     def hasNext(self):\n#         \"\"\"\n#         Returns true if the iteration has more elements.\n#         :rtype: bool\n#         \"\"\"\n#\n#     def next(self):\n#         \"\"\"\n#         Returns the next element in the iteration.\n#         :rtype: int\n#         \"\"\"\n\nclass PeekingIterator:\n    def __init__(self, iterator):\n        \"\"\"\n        Initialize your data structure here.\n        :type iterator: Iterator\n        \"\"\"\n        \n\n    def peek(self):\n        \"\"\"\n        Returns the next element in the iteration without advancing the iterator.\n        :rtype: int\n        \"\"\"\n        \n\n    def next(self):\n        \"\"\"\n        :rtype: int\n        \"\"\"\n        \n\n    def hasNext(self):\n        \"\"\"\n        :rtype: bool\n        \"\"\"\n        \n\n# Your PeekingIterator object will be instantiated and called as such:\n# iter = PeekingIterator(Iterator(nums))\n# while iter.hasNext():\n#     val = iter.peek()   # Get the next element but not advance the iterator.\n#     iter.next()         # Should return the same value as [val]."}, {"value": "c", "text": "C", "defaultCode": "/*\n *\tstruct Iterator {\n *\t\t// Returns true if the iteration has more elements.\n *\t\tbool (*hasNext)();\n *\n * \t\t// Returns the next element in the iteration.\n *\t\tint (*next)();\n *\t};\n */\n\nstruct PeekingIterator {\n    \n};\n\nstruct PeekingIterator* Constructor(struct Iterator* iter) {\n    struct PeekingIterator* piter = malloc(sizeof(struct PeekingIterator));\n    piter->iterator = iter;\n    piter->hasPeeked = false;\n    return piter;\n}\n\nint peek(struct PeekingIterator* obj) {\n    \n}\n\nint next(struct PeekingIterator* obj) {\n    \n}\n\nbool hasNext(struct PeekingIterator* obj) {\n    \n}\n\n/**\n * Your PeekingIterator struct will be instantiated and called as such:\n * PeekingIterator* obj = peekingIteratorCreate(arr, arrSize);\n * int param_1 = peek(obj);\n * int param_2 = next(obj);\n * bool param_3 = hasNext(obj);\n * peekingIteratorFree(obj);\n*/"}, {"value": "csharp", "text": "C#", "defaultCode": "// C# IEnumerator interface reference:\n// https://docs.microsoft.com/en-us/dotnet/api/system.collections.ienumerator?view=netframework-4.8\n\nclass PeekingIterator {\n    // iterators refers to the first element of the array.\n    public PeekingIterator(IEnumerator<int> iterator) {\n        // initialize any member here.\n    }\n    \n    // Returns the next element in the iteration without advancing the iterator.\n    public int Peek() {\n        \n    }\n    \n    // Returns the next element in the iteration and advances the iterator.\n    public int Next() {\n        \n    }\n    \n    // Returns false if the iterator is refering to the end of the array of true otherwise.\n    public bool HasNext() {\n\t\t\n    }\n}"}, {"value": "javascript", "text": "JavaScript", "defaultCode": "/**\n * // This is the Iterator's API interface.\n * // You should not implement it, or speculate about its implementation.\n * function Iterator() {\n *    @ return {number}\n *    this.next = function() { // return the next number of the iterator\n *       ...\n *    }; \n *\n *    @return {boolean}\n *    this.hasNext = function() { // return true if it still has numbers\n *       ...\n *    };\n * };\n */\n\n/**\n * @param {Iterator} iterator\n */\nvar PeekingIterator = function(iterator) {\n    \n};\n\n/**\n * @return {number}\n */\nPeekingIterator.prototype.peek = function() {\n    \n};\n\n/**\n * @return {number}\n */\nPeekingIterator.prototype.next = function() {\n    \n};\n\n/**\n * @return {boolean}\n */\nPeekingIterator.prototype.hasNext = function() {\n    \n};\n\n/** \n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = new PeekingIterator(arr)\n * var param_1 = obj.peek()\n * var param_2 = obj.next()\n * var param_3 = obj.hasNext()\n */"}, {"value": "typescript", "text": "TypeScript", "defaultCode": "/**\n * // This is the Iterator's API interface.\n * // You should not implement it, or speculate about its implementation\n * class Iterator {\n *      hasNext(): boolean {}\n *\n *      next(): number {}\n * }\n */\n\nclass PeekingIterator {\n    constructor(iterator: Iterator) {\n\n    }\n\n    peek(): number {\n\n    }\n\n    next(): number {\n\n    }\n\n    hasNext(): boolean {\n\n    }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = new PeekingIterator(iterator)\n * var param_1 = obj.peek()\n * var param_2 = obj.next()\n * var param_3 = obj.hasNext()\n */"}, {"value": "php", "text": "PHP", "defaultCode": "// PHP ArrayIterator reference:\n// https://www.php.net/arrayiterator\n\nclass PeekingIterator {\n    /**\n     * @param ArrayIterator $arr\n     */\n    function __construct($arr) {\n        \n    }\n    \n    /**\n     * @return Integer\n     */\n    function next() {\n        \n    }\n    \n    /**\n     * @return Integer\n     */\n    function peek() {\n        \n    }\n    \n    /**\n     * @return Boolean\n     */\n    function hasNext() {\n        \n    }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * $obj = PeekingIterator($arr);\n * $ret_1 = $obj->next();\n * $ret_2 = $obj->peek();\n * $ret_3 = $obj->hasNext();\n */"}, {"value": "swift", "text": "Swift", "defaultCode": "// Swift IndexingIterator refernence:\n// https://developer.apple.com/documentation/swift/indexingiterator\n\nclass PeekingIterator {\n    init(_ arr: IndexingIterator<Array<Int>>) {\n        \n    }\n    \n    func next() -> Int {\n        \n    }\n    \n    func peek() -> Int {\n        \n    }\n    \n    func hasNext() -> Bool {\n        \n    }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * let obj = PeekingIterator(arr)\n * let ret_1: Int = obj.next()\n * let ret_2: Int = obj.peek()\n * let ret_3: Bool = obj.hasNext()\n */"}, {"value": "kotlin", "text": "Kotlin", "defaultCode": "// Kotlin Iterator reference:\n// https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-iterator/\n\nclass PeekingIterator(iterator:Iterator<Int>):Iterator<Int> {\n    fun peek(): Int {\n    \t\n    }\n    \n    override fun next(): Int {\n        \n    }\n    \n    override fun hasNext(): Boolean {\n        \n    }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = PeekingIterator(arr)\n * var param_1 = obj.next()\n * var param_2 = obj.peek()\n * var param_3 = obj.hasNext()\n */"}, {"value": "golang", "text": "Go", "defaultCode": "/*   Below is the interface for Iterator, which is already defined for you.\n *\n *   type Iterator struct {\n *       \n *   }\n *\n *   func (this *Iterator) hasNext() bool {\n *\t\t// Returns true if the iteration has more elements.\n *   }\n *\n *   func (this *Iterator) next() int {\n *\t\t// Returns the next element in the iteration.\n *   }\n */\n\ntype PeekingIterator struct {\n    \n}\n\nfunc Constructor(iter *Iterator) *PeekingIterator {\n    \n}\n\nfunc (this *PeekingIterator) hasNext() bool {\n    \n}\n\nfunc (this *PeekingIterator) next() int {\n    \n}\n\nfunc (this *PeekingIterator) peek() int {\n    \n}"}, {"value": "ruby", "text": "Ruby", "defaultCode": "# Below is the interface for Iterator, which is already defined for you.\n#\n# class Iterator\n# \tdef initialize(v)\n#   \n#   end\n#\n#   def hasNext()\n#\t\tReturns true if the iteration has more elements.\n#   end\n#\n#   def next()\n#   \tReturns the next element in the iteration.\n#   end\n# end\n\nclass PeekingIterator\n    # @param {Iterator} iter\n    def initialize(iter)\n    \t\n    end\n    \n    # Returns true if the iteration has more elements.\n    # @return {boolean}\n    def hasNext()\n    \t\n    end\n    \n    # Returns the next element in the iteration.\n    # @return {integer}\n    def next()\n    \t\n    end\n    \n    # Returns the next element in the iteration without advancing the iterator.\n    # @return {integer}\n    def peek()\n    \t\n    end\nend"}, {"value": "scala", "text": "Scala", "defaultCode": "// Scala Iterator reference:\n// https://www.scala-lang.org/api/2.12.2/scala/collection/Iterator.html\n\nclass PeekingIterator(_iterator: Iterator[Int]) {\n    def peek(): Int = {\n        \n    }\n    \n    def next(): Int = {\n        \n    }\n    \n    def hasNext(): Boolean = {\n        \n    }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = new PeekingIterator(arr)\n * var param_1 = obj.next()\n * var param_2 = obj.peek()\n * var param_3 = obj.hasNext()\n */"}]
{
  "name": "PeekingIterator",
  "params": [
    {
      "name": "nums",
      "type": "list<integer>"
    },
    {
      "name": "commands",
      "type": "list<integer>"
    }
  ],
  "return": {
    "type": "list<string>",
    "dealloc": true
  },
  "languages": [
    "cpp",
    "java",
    "python",
    "python3",
    "csharp",
    "golang",
    "ruby",
    "kotlin",
    "scala",
    "swift",
    "php",
    "c",
    "javascript",
    "typescript"
  ],
  "manual": true,
 "typescriptCustomType" : "class Iterator {\n    data: number[];\n    index: number;\n    constructor(data: number[]) {\n        this.data = data;\n        this.index = 0;\n    }\n\n    hasNext(): boolean {\n        return this.index != this.data.length;\n    }\n\n    next(): number {\n        var ret = -100000000;\n        if (this.hasNext()) {\n            ret = this.data[this.index];\n            this.index++;\n        }\n        return ret;\n    }\n};\n" 
}
{"cpp": ["C++", "<p>Compiled with <code> clang 19 </code> using the latest C++ 23 standard, and <code>libstdc++</code> provided by GCC 14.</p>\r\n\r\n<p>Your code is compiled with level two optimization (<code>-O2</code>). <a href=\"https://github.com/google/sanitizers/wiki/AddressSanitizer\" target=\"_blank\">AddressSanitizer</a> is also enabled to help detect out-of-bounds and use-after-free bugs.</p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>"], "java": ["Java", "<p><code>OpenJDK 21</code>. Using compile arguments: <code>--enable-preview --release 21</code></p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>\r\n<p>Includes <code>Pair</code> class from https://docs.oracle.com/javase/8/javafx/api/javafx/util/Pair.html.</p>"], "python": ["Python", "<p><code>Python 2.7.18</code>.</p>\r\n\r\n<p>Most libraries are already imported automatically for your convenience, such as <a href=\"https://docs.python.org/2/library/array.html\" target=\"_blank\">array</a>, <a href=\"https://docs.python.org/2/library/bisect.html\" target=\"_blank\">bisect</a>, <a href=\"https://docs.python.org/2/library/collections.html\" target=\"_blank\">collections</a>. If you need more libraries, you can import it yourself.</p>\r\n\r\n<p>For Map/TreeMap data structure, you may use <a href=\"http://www.grantjenks.com/docs/sortedcontainers/\" target=\"_blank\">sortedcontainers</a> library.</p>\r\n\r\n<p>Note that Python 2.7 <a href=\"https://www.python.org/dev/peps/pep-0373/\" target=\"_blank\">is not maintained anymore</a>. For the latest Python, please choose Python3 instead.</p>"], "c": ["C", "<p>Compiled with <code>gcc 14</code> using the gnu11 standard.</p>\r\n\r\n<p>Your code is compiled with level one optimization (<code>-O2</code>). <a href=\"https://github.com/google/sanitizers/wiki/AddressSanitizer\" target=\"_blank\">AddressSanitizer</a> is also enabled to help detect out-of-bounds and use-after-free bugs.</p>\r\n\r\n<p>Most standard library headers are already included automatically for your convenience.</p>\r\n\r\n<p>For hash table operations, you may use <a href=\"https://troydhanson.github.io/uthash/\" target=\"_blank\">uthash</a>. \"uthash.h\" is included by default. Below are some examples:</p>\r\n\r\n<p><b>1. Adding an item to a hash.</b>\r\n<pre>\r\nstruct hash_entry {\r\n    int id;            /* we'll use this field as the key */\r\n    char name[10];\r\n    UT_hash_handle hh; /* makes this structure hashable */\r\n};\r\n\r\nstruct hash_entry *users = NULL;\r\n\r\nvoid add_user(struct hash_entry *s) {\r\n    HASH_ADD_INT(users, id, s);\r\n}\r\n</pre>\r\n</p>\r\n\r\n<p><b>2. Looking up an item in a hash:</b>\r\n<pre>\r\nstruct hash_entry *find_user(int user_id) {\r\n    struct hash_entry *s;\r\n    HASH_FIND_INT(users, &user_id, s);\r\n    return s;\r\n}\r\n</pre>\r\n</p>\r\n\r\n<p><b>3. Deleting an item in a hash:</b>\r\n<pre>\r\nvoid delete_user(struct hash_entry *user) {\r\n    HASH_DEL(users, user);  \r\n}\r\n</pre>\r\n</p>"], "csharp": ["C#", "<p><a href=\"https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13\" target=\"_blank\">C# 13</a> with .NET 9 runtime</p>"], "javascript": ["JavaScript", "<p><code>Node.js 22.14.0</code>.</p>\r\n\r\n<p>Your code is run with <code>--harmony</code> flag, enabling <a href=\"http://node.green/\" target=\"_blank\">new ES6 features</a>.</p>\r\n\r\n<p><a href=\"https://lodash.com\" target=\"_blank\">lodash.js</a> library is included by default.</p>\r\n\r\n<p>For Priority Queue / Queue data structures, you may use 6.3.2 version of <a href=\"https://github.com/datastructures-js/priority-queue/tree/v6.3.2\" target=\"_blank\">datastructures-js/priority-queue</a> and 4.2.3 version of <a href=\"https://github.com/datastructures-js/queue/tree/v4.2.3\" target=\"_blank\">datastructures-js/queue</a> and 1.0.4 version of <a href=\"https://github.com/datastructures-js/deque/tree/v1.0.4\" target=\"_blank\">datastructures-js/deque</a>.</p>"], "ruby": ["Ruby", "<p><code>Ruby 3.2</code></p>\r\n\r\n<p>Some common data structure implementations are provided in the Algorithms module: https://www.rubydoc.info/github/kanwei/algorithms/Algorithms</p>"], "swift": ["Swift", "<p><code>Swift 6.0</code>.</p>\r\n\r\n<p>You may use the following packages:<br/>\r\n<a href=\"https://github.com/apple/swift-algorithms/tree/1.2.0\" target=\"_blank\">swift-algorithms 1.2.0</a><br/>\r\n<a href=\"https://github.com/apple/swift-collections/tree/1.1.4\" target=\"_blank\">swift-collections 1.1.4</a><br/>\r\n<a href=\"https://github.com/apple/swift-numerics/tree/1.0.2\" target=\"_blank\">swift-numerics 1.0.2</a></p>"], "golang": ["Go", "<p><code>Go 1.23</code></p>\r\n<p>Support <a href=\"https://pkg.go.dev/github.com/emirpasic/gods@v1.18.1\" target=\"_blank\">https://pkg.go.dev/github.com/emirpasic/gods@v1.18.1</a> and <a href=\"https://pkg.go.dev/github.com/emirpasic/gods/v2@v2.0.0-alpha\" target=\"_blank\">https://pkg.go.dev/github.com/emirpasic/gods/v2@v2.0.0-alpha</a> library.</p>"], "python3": ["Python3", "<p><code>Python 3.11</code>.</p>\r\n\r\n<p>Most libraries are already imported automatically for your convenience, such as <a href=\"https://docs.python.org/3/library/array.html\" target=\"_blank\">array</a>, <a href=\"https://docs.python.org/3/library/bisect.html\" target=\"_blank\">bisect</a>, <a href=\"https://docs.python.org/3/library/collections.html\" target=\"_blank\">collections</a>. If you need more libraries, you can import it yourself.</p>\r\n\r\n<p>For Map/TreeMap data structure, you may use <a href=\"http://www.grantjenks.com/docs/sortedcontainers/\" target=\"_blank\">sortedcontainers</a> library.</p>"], "scala": ["Scala", "<p><code>Scala 3.3.1</code>.</p>"], "kotlin": ["Kotlin", "<p><code>Kotlin 2.1.10</code>.</p>"], "php": ["PHP", "<p><code>PHP 8.2</code>.</p>\r\n<p>With bcmath module</p>"], "typescript": ["Typescript", "<p><code>TypeScript 5.7.3, Node.js 22.14.0</code>.</p>\r\n\r\n<p>Compile Options: <code>--alwaysStrict --strictBindCallApply --strictFunctionTypes --target ES2024</code></p>\r\n\r\n<p><a href=\"https://lodash.com\" target=\"_blank\">lodash.js</a> library is included by default.</p>\r\n\r\n<p>For Priority Queue / Queue data structures, you may use 6.3.2 version of <a href=\"https://github.com/datastructures-js/priority-queue/tree/v6.3.2\" target=\"_blank\">datastructures-js/priority-queue</a> and 4.2.3 version of <a href=\"https://github.com/datastructures-js/queue/tree/v4.2.3\" target=\"_blank\">datastructures-js/queue</a> and 1.0.4 version of <a href=\"https://github.com/datastructures-js/deque/tree/v1.0.4\" target=\"_blank\">datastructures-js/deque</a>.</p>"]}
['Array', 'Design', 'Iterator']