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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

• Java
• C++
• Python
• Go
• TypeScript
• Javascript

• 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;
      }
  };
  

• 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
  
  

• 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;
  };
  
  

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