1991. Find the Middle Index in Array

#### QUESTION:

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
``````

#### SOLUTION:

``````class Solution {
func findMiddleIndex(_ nums: [Int]) -> Int {
var left:[Int] = [0]
var right:[Int] = [0]
var sum:Int = 0
for num in nums {
left.append(sum+num)
sum += num
}
sum = 0
for num in nums.reversed() {
right.insert(sum+num, at: 0)
sum += num
}
for index in 0...nums.count - 1 {
if left[index] == right[index + 1] {
return index
}
}
return -1
}
}
``````
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