David Lowes

Equal Sides Of An Array

You are going to be given an array of integers. Your job is to take that array and find an index N where the sum of the integers to the left of N is equal to the sum of the integers to the right of N. If there is no index that would make this happen, return -1.

For Example
Let's say you are given the array {1,2,3,4,3,2,1}:
Your function will return the index 3, because at the 3rd position of the array, the sum of left side of the index ({1,2,3}) and the sum of the right side of the index ({3,2,1}) both equal 6.
You are given the array {1,100,50,-51,1,1}:
Your function will return the index 1, because at the 1st position of the array, the sum of left side of the index ({1}) and the sum of the right side of the index ({50,-51,1,1}) both equal 1.
You are given the array {20,10,-80,10,10,15,35}
At index 0 the left side is {}
The right side is {10,-80,10,10,15,35}
They both are equal to 0 when added. (Empty arrays are equal to 0 in this problem)
Index 0 is the place where the left side and right side are equal.

NOTE: Array index starts at 0 in Javascript

Input format
An integer array of length 0 < arr < 1000. The numbers in the array can be any integer positive or negative.

Output format
The lowest index N where the side to the left of N is equal to the side to the right of N. If you do not find an index that fits these rules, then you will return -1.

Solution

function findEvenIndex(arr) {
  var sum = 0,
  leftSum = 0;
  // add all ints first
  for (var i = 0; i < arr.length; i++) {
    sum = sum + arr[i];
  }
  // subtract all ints so you can find the middle with an exit clause if statement
  for (var i = 0; i < arr.length; i++) {
    sum = sum - arr[i];
    if (leftSum === sum) {
      return i;
    }

    leftSum = leftSum + arr[i];
  }

  return -1;
}