FindPivotInteger.php

<?php

namespace App;

/**
 * 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.
 *
 * https://leetcode.com/problems/find-the-pivot-integer
 */
class FindPivotInteger
{
    /**
     * @param int $n
     * @return int
     */
    public function pivotInteger(int $n): int
    {
        $sum = 0;
        for ($i = 1; $i <= $n; $i++) {
            $sum += $i;
        }

        $leftSum = 0;
        for ($i = 1; $i <= $n; $i++) {
            if ($leftSum === $sum - $leftSum - $i) {
                return $i;
            }

            $leftSum += $i;
        }

        return -1;
    }
}