g2401_2500.s2485_find_the_pivot_integer.Solution Maven / Gradle / Ivy

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Java-based LeetCode algorithm problem solutions, regularly updated

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package g2401_2500.s2485_find_the_pivot_integer;

// #Easy #Math #Prefix_Sum #2023_01_26_Time_1_ms_(95.67%)_Space_39.2_MB_(91.00%)

/**
 * 2485 - Find the Pivot Integer\.
 *
 * Easy
 *
 * 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`
**/
public class Solution {
    public int pivotInteger(int n) {
        if (n == 0 || n == 1) {
            return n;
        }
        int sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += i;
        }
        int ad = 0;
        for (int i = 1; i <= n; i++) {
            ad += i - 1;
            sum -= i;
            if (sum == ad) {
                return i;
            }
        }
        return -1;
    }
}