Today I have done leetcode's March LeetCoding Challenge with cpp.

March LeetCoding Challenge 13

Description

Find the Pivot Integer

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

// ((1 + n) * n / 2 + x) / 2 = (1 + x) * x / 2
// n^2 / 2 + n / 2 += x ^ 2
class Solution {
public:
  int pivotInteger(int n) {
    int target = (n * (n + 1)) / 2;
    int x = sqrt(target);
    if(x * x == target) return x;
    return -1;
  }
};