solution.js

function solution(A) {
  var resultsArray = [];

  var i;

  for (i = 1; i < A.length - 1; i++) {
    var leftArray = [];
    var rigthArray = [];


    for (var l = i - 1; l > -1; l--) {
      leftArray.push(A[l]);
    }

    for (var r = i+1; r < A.length; r++) {
      rigthArray.push(A[r]);
    }

    var currentLeftSum = leftArray.reduce(add, 0);
    var currentRightSum = rigthArray.reduce(add, 0);

    if (currentLeftSum === currentRightSum) {
      resultsArray.push(i)
    }
  }
  return resultsArray;
}

function add(a, b) {
  return a + b;
}

solution([-1, 3, -4, 5, 1, -6, 2, 1])

task.md

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 < 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.

For example, consider the following array A consisting of N = 8 elements:

A[0] = -1 A[1] = 3 A[2] = -4 A[3] = 5 A[4] = 1 A[5] = -6 A[6] = 2 A[7] = 1 P = 1 is an equilibrium index of this array, because:

A[0] = −1 = A[2] + A[3] + A[4] + A[5] + A[6] + A[7] P = 3 is an equilibrium index of this array, because:

A[0] + A[1] + A[2] = −2 = A[4] + A[5] + A[6] + A[7] P = 7 is also an equilibrium index, because:

A[0] + A[1] + A[2] + A[3] + A[4] + A[5] + A[6] = 0 and there are no elements with indices greater than 7.

P = 8 is not an equilibrium index, because it does not fulfill the condition 0 ≤ P < N.

Write a function:

function solution(A); 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.

For example, given array A shown above, the function may return 1, 3 or 7, as explained above.

Assume that:

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]. Complexity:

expected worst-case time complexity is O(N); expected worst-case space complexity is O(N), beyond input storage (not counting the storage required for input arguments). Elements of input arrays can be modified.