LeetCode 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.

Constraints:

• 1 <= nums.length <= 100

• -1000 <= nums[i] <= 1000

 presum 求解即可，只是没想到会100%。。。。

Runtime: 0 ms, faster than 100.00% of Java online submissions for Find the Middle Index in Array.

Memory Usage: 41.3 MB, less than 33.54% of Java online submissions for Find the Middle Index in Array.