Find the pivot integer

Math Prefix Sum

83.8% acceptance

Feb 25, 2026

1432

Given a positive integer n, find the pivot integer x such that: sum(1..=x) == sum(x..=n) Return x, or -1 if no such integer exists.

Solution

Rust

Time O(1)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn pivot_integer(n: i32) -> i32 {
    // x^2 = n*(n+1)/2
    let total = n * (n + 1) / 2;
    let x = (total as f64).sqrt() as i32;
    if x * x == total { x } else { -1 }
  }
}