724. Find Pivot Index

Given array nums, return the leftmost pivot index where sum of elements to the left equals sum of elements to the right. Return -1 if none exists.

Approach: compute total sum upfront, then walk left to right tracking running left sum; right sum derived as total - left_sum - nums[i].

#![allow(unused)]
fn main() {
// time O(n), space O(1)
pub fn pivot_index(nums: Vec<i32>) -> i32 {
    let total: i32 = nums.iter().sum();
    let mut left_sum = 0;
    for i in 0..nums.len() {
        let right_sum = total - left_sum - nums[i];
        if left_sum == right_sum {
            return i as i32;
        }
        left_sum += nums[i];
    }

    -1
}
}