1991. 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.

Example 4:

Input: nums = [1]
Output: 0
Explantion:
The sum of the numbers before index 0 is: 0
The sum of the numbers after index 0 is: 0

 

Constraints:

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

 

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

1991. Find the Middle Index in Array
struct Solution;

impl Solution {
    fn find_middle_index(nums: Vec<i32>) -> i32 {
        let n = nums.len();
        for i in 0..n {
            let left: i32 = (0..i).map(|j| nums[j]).sum();
            let right: i32 = (i + 1..n).map(|j| nums[j]).sum();
            if left == right {
                return i as i32;
            }
        }
        -1
    }
}

#[test]
fn test() {
    let nums = vec![2, 3, -1, 8, 4];
    let res = 3;
    assert_eq!(Solution::find_middle_index(nums), res);
    let nums = vec![1, -1, 4];
    let res = 2;
    assert_eq!(Solution::find_middle_index(nums), res);
    let nums = vec![2, 5];
    let res = -1;
    assert_eq!(Solution::find_middle_index(nums), res);
    let nums = vec![1];
    let res = 0;
    assert_eq!(Solution::find_middle_index(nums), res);
}