Add digits

Math Simulation Number Theory

68.6% acceptance

Jan 12, 2026

5465

1978

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Solution

Rust

Time O(1)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn add_digits(num: i32) -> i32 {
    if num == 0 {
      0
    } else if num % 9 == 0 {
      9
    } else {
      num % 9
    }
  }
}