2485. Find the Pivot Integer

#### QUESTION:

Given a positive integer n, find the pivot integer x such that:

The sum of all elements between 1 and x inclusively equals the sum of all elements between x and n inclusively. Return the pivot integer x. If no such integer exists, return -1. It is guaranteed that there will be at most one pivot index for the given input.

Example 1:

``````Input: n = 8
Output: 6
Explanation: 6 is the pivot integer since: 1 + 2 + 3 + 4 + 5 + 6 = 6 + 7 + 8 = 21.
``````

Example 2:

``````Input: n = 1
Output: 1
Explanation: 1 is the pivot integer since: 1 = 1.
``````

Example 3:

``````Input: n = 4
Output: -1
Explanation: It can be proved that no such integer exist.
``````

Constraints:

``````1 <= n <= 1000
``````

#### SOLUTION:

``````class Solution {
func pivotInteger(_ n: Int) -> Int {
var result = -1
var preSum = 0
var postSum = 0
for index in 1...n {
postSum += index
}
for index in 1...n {
preSum += index
postSum -= index-1
if (preSum == postSum) {
return index
}
}
return result
}
}
``````