Approach: Prefix sum

def pivotIndex(nums):
    total = sum(nums)
    left = 0
    for i, x in enumerate(nums):
        if left == total - left - x:
            return i
        left += x
    return -1

Complexity: O(n) time, O(1) space.

Full Solution — all languages

class Solution:
    def pivotIndex(self, nums):
        total = sum(nums)
        left = 0
        for i, x in enumerate(nums):
            if left == total - left - x:
                return i
            left += x
        return -1

import java.util.*;
class Solution {
    public int pivotIndex(int[] nums) {
        int total = 0;
        for (int x : nums) total += x;
        int left = 0;
        for (int i = 0; i < nums.length; i++) {
            if (left == total - left - nums[i]) return i;
            left += nums[i];
        }
        return -1;
    }
}

#include <bits/stdc++.h> using namespace std;
class Solution {
    public: int pivotIndex(vector<int>& nums) {
        int total = 0;
        for (int x : nums) total += x;
        int left = 0;
        for (int i = 0; i < (int)nums.size(); i++) {
            if (left == total - left - nums[i]) return i;
            left += nums[i];
        }
        return -1;
    }
};

/**
 * @param {number[]} nums
 * @return {number}
 */ var pivotIndex = function(nums) {
    let total = 0;
    for (const x of nums) total += x;
    let left = 0;
    for (let i = 0; i < nums.length; i++) {
        if (left === total - left - nums[i]) return i;
        left += nums[i];
    }
    return -1;
};

Find Pivot Index

Example 1

Example 2

Example 3

Constraints