title: A zero-indexed array given & An equilibrium index of this array
body: <p>A zero-indexed array A consisting of N integers is given. An equilibrium index of this array is any integer P such that 0 ≤ P &lt; N and the sum of elements of lower indices is equal to the sum of elements of higher indices, i.e. 
  A[0] + A[1] + ... + A[P−1] = A[P+1] + ... + A[N−2] + A[N−1].
  Sum of zero elements is assumed to be equal to 0. This can happen if P = 0 or if P = N−1.</p>
  
  <p>For example, consider the following array A consisting of N = 8 elements:</p>
  
  <pre><code>  A[0] = -1
    A[1] =  3
    A[2] = -4
    A[3] =  5
    A[4] =  1
    A[5] = -6
    A[6] =  2
    A[7] =  1
  </code></pre>
  
  <p>P = 1 is an equilibrium index of this array, because:</p>
  
  <pre><code>A[0] = −1 = A[2] + A[3] + A[4] + A[5] + A[6] + A[7]
  </code></pre>
  
  <p>P = 3 is an equilibrium index of this array, because:</p>
  
  <pre><code>A[0] + A[1] + A[2] = −2 = A[4] + A[5] + A[6] + A[7]
  </code></pre>
  
  <p>P = 7 is also an equilibrium index, because:</p>
  
  <pre><code>A[0] + A[1] + A[2] + A[3] + A[4] + A[5] + A[6] = 0
  </code></pre>
  
  <p>and there are no elements with indices greater than 7.</p>
  
  <p>P = 8 is not an equilibrium index, because it does not fulfill the condition 0 ≤ P &lt; N.</p>
  
  <p>Now i have to write a function:</p>
  
  <pre><code>int solution(int A[], int N);
  </code></pre>
  
  <p>that, given a zero-indexed array A consisting of N integers, returns any of its equilibrium indices. The function should return −1 if no equilibrium index exists.</p>
  
  <p>For example, given array A shown above, the function may return 1, 3 or 7, as explained above.</p>
  
  <p>Assume that:</p>
  
  <pre><code>N is an integer within the range [0..100,000];
  each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].
  </code></pre>
  
  <p>here have some Complexity:</p>
  
  <pre><code>Elements of input arrays can be modified.
  </code></pre>