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#1991 - Find the Middle Index in Array
Problem Description
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.
Solution
/**
* @param {number[]} nums
* @return {number}
*/
var findMiddleIndex = function(nums) {
let left = 0;
let right = nums.reduce((sum, n) => sum + n, 0);
for (let i = 0; i < nums.length; i++) {
left += nums[i];
right -= nums[i];
if (left - nums[i] === right) {
return i;
}
}
return -1;
};