1991. Find the Middle Index in Array LeetCode Solution

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Table of Contents

1. Problem Statement
2. Complexity Analysis
3. Find the Middle Index in Array solution in C++
4. Find the Middle Index in Array solution in Java
5. Find the Middle Index in Array solution in Python
6. Additional Resources

Problem Statement of 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.

Constraints:

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

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

Complexity Analysis

• Time Complexity: O(n)
• Space Complexity: O(1)

1991. Find the Middle Index in Array LeetCode Solution in C++

class Solution {
 public:
  int findMiddleIndex(vector<int>& nums) {
    int prefix = 0;
    int suffix = accumulate(nums.begin(), nums.end(), 0);

    for (int i = 0; i < nums.size(); ++i) {
      suffix -= nums[i];
      if (prefix == suffix)
        return i;
      prefix += nums[i];
    }

    return -1;
  }
};
/* code provided by PROGIEZ */

1991. Find the Middle Index in Array LeetCode Solution in Java

class Solution {
  public int findMiddleIndex(int[] nums) {
    int prefix = 0;
    int suffix = Arrays.stream(nums).sum();

    for (int i = 0; i < nums.length; ++i) {
      suffix -= nums[i];
      if (prefix == suffix)
        return i;
      prefix += nums[i];
    }

    return -1;
  }
}
// code provided by PROGIEZ

1991. Find the Middle Index in Array LeetCode Solution in Python

class Solution:
  def findMiddleIndex(self, nums: list[int]) -> int:
    prefix = 0
    suffix = sum(nums)

    for i, num in enumerate(nums):
      suffix -= num
      if prefix == suffix:
        return i
      prefix += num

    return -1
 # code by PROGIEZ

Additional Resources

• Explore all LeetCode problem solutions at Progiez here
• Explore all problems on LeetCode website here

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