Question

1991. Find the Middle Index in Array

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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

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

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;
  }
}

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;
  }
};

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
}

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;
}

/**
 * @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;
};