#2485 - Find the Pivot Integer

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Problem Description

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.

Solution

/**
 * @param {number} n
 * @return {number}
 */
var pivotInteger = function(n) {
  const totalSum = n * (n + 1) / 2;
  const pivot = Math.sqrt(totalSum);
  return Number.isInteger(pivot) && pivot <= n ? pivot : -1;
};