In [THIS](http://www.geeksforgeeks.org/level-order-tree-traversal/) geeksforgeeks link they are describing the time complexity of recursive level order traversal as `O(n^2)`.

Time Complexity: `O(n^2)` in worst case. For a skewed tree, `printGivenLevel()` takes `O(n)` time where n is the number of nodes in the skewed tree. So time complexity of `printLevelOrder()` is `O(n) + O(n-1) + O(n-2) + .. + O(1)` which is `O(n^2)`.This is not clear to me. 

Can someone please help me understand.