Calculate digit sum of a string

String Simulation

67.7% acceptance

Feb 25, 2026

595

You are given a string s consisting of digits and an integer k. A round can be completed if the length of s is greater than k. In one round, do the following: Divide s into consecutive groups of size k such that the first k characters are in the first group, the next k characters are in the second group, and so on. Note that the size of the last group can be smaller than k. Replace each group of s with a string representing the sum of all its digits. For example, "346" is replaced with "13" because 3 + 4 + 6 = 13. Merge consecutive groups together to form a new string. If the length of the string is greater than k, repeat from step 1. Return s after all rounds have been completed.

Solution

Rust

Time O(n²)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn digit_sum(s: String, k: i32) -> String {
    let k = k as usize;
    let mut s = s;
    while s.len() > k {
      let chars: Vec<char> = s.chars().collect();
      let mut new_s = String::new();
      for chunk in chars.chunks(k) {
        let sum: u32 = chunk.iter().map(|&c| c as u32 - '0' as u32).sum();
        new_s.push_str(&sum.to_string());
      }
      s = new_s;
    }
    s
  }
}