FindMiddleIndexInArray.php

<?php

namespace App;

/**
 * Find the Middle Index in Array
 *
 * Given a 0-indexed integer array nums, find the leftmost middleIndex (i.e., the smallest amongst all the possible
 * ones). A middleIndex is an index where nums[0] + nums[1] + ... + nums[middleIndex-1] == nums[middleIndex+1] +
 * nums[middleIndex+2] + ... + nums[nums.length-1]. If middleIndex == 0, the left side sum is considered to be 0.
 * Similarly, if middleIndex == nums.length - 1, the right side sum is considered to be 0. Return the leftmost
 * middleIndex that satisfies the condition, or -1 if there is no such index.
 *
 * Example 1:
 * Input: nums = [2,3,-1,8,4]
 * Output: 3
 * Explanation: The sum of the numbers before index 3 is: 2 + 3 + -1 = 4
 * The sum of the numbers after index 3 is: 4 = 4
 *
 * Example 2:
 * Input: nums = [1,-1,4]
 * Output: 2
 * Explanation: The sum of the numbers before index 2 is: 1 + -1 = 0
 * The sum of the numbers after index 2 is: 0
 *
 * Example 3:
 * Input: nums = [2,5]
 * Output: -1
 * Explanation: There is no valid middleIndex.
 *
 * https://leetcode.com/problems/find-the-middle-index-in-array
 */
class FindMiddleIndexInArray
{
    /**
     * @param int[] $nums
     * @return int
     */
    public function findMiddleIndex(array $nums): int
    {
        $sum = array_sum($nums);
        $leftSum = 0;
        /** @var int $index */
        foreach ($nums as $index => $num) {
            if ($leftSum === ($sum - $leftSum - $num)) {
                return $index;
            }
            $leftSum += $num;
        }
        return -1;
    }
}