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1991. Find the Middle Index in Array

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.

 

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.

 

Constraints:

  • 1 <= nums.length <= 100
  • -1000 <= nums[i] <= 1000

 

Note: This question is the same as 724: https://leetcode.com/problems/find-pivot-index/

Solutions

Solution 1

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class Solution:
    def findMiddleIndex(self, nums: List[int]) -> int:
        left, right = 0, sum(nums)
        for i, x in enumerate(nums):
            right -= x
            if left == right:
                return i
            left += x
        return -1

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class Solution {
    public int findMiddleIndex(int[] nums) {
        int left = 0, right = Arrays.stream(nums).sum();
        for (int i = 0; i < nums.length; ++i) {
            right -= nums[i];
            if (left == right) {
                return i;
            }
            left += nums[i];
        }
        return -1;
    }
}

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class Solution {
public:
    int findMiddleIndex(vector<int>& nums) {
        int left = 0, right = accumulate(nums.begin(), nums.end(), 0);
        for (int i = 0; i < nums.size(); ++i) {
            right -= nums[i];
            if (left == right) {
                return i;
            }
            left += nums[i];
        }
        return -1;
    }
};

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func findMiddleIndex(nums []int) int {
    s := 0
    for _, num := range nums {
        s += num
    }
    total := 0
    for i, num := range nums {
        total += num
        if total-num == s-total {
            return i
        }
    }
    return -1
}

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function findMiddleIndex(nums: number[]): number {
    let left = 0,
        right = nums.reduce((a, b) => a + b);
    for (let i = 0; i < nums.length; ++i) {
        right -= nums[i];
        if (left == right) {
            return i;
        }
        left += nums[i];
    }
    return -1;
}

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/**
 * @param {number[]} nums
 * @return {number}
 */
var findMiddleIndex = function (nums) {
    let left = 0,
        right = nums.reduce((a, b) => a + b);
    for (let i = 0; i < nums.length; ++i) {
        right -= nums[i];
        if (left == right) {
            return i;
        }
        left += nums[i];
    }
    return -1;
};

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