Count largest group

Hash Table Math Counting

74.7% acceptance

Feb 25, 2026

802

1190

You are given an integer n. We need to group the numbers from 1 to n according to the sum of its digits. For example, the numbers 14 and 5 belong to the same group, whereas 13 and 3 belong to different groups. Return the number of groups that have the largest size, i.e. the maximum number of elements.

Solution

Rust

Time O(n²)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn count_largest_group(n: i32) -> i32 {
    let mut groups = std::collections::HashMap::new();
    for i in 1..=n {
      let mut x = i;
      let mut s = 0;
      while x > 0 { s += x % 10; x /= 10; }
      *groups.entry(s).or_insert(0) += 1;
    }
    let max_size = *groups.values().max().unwrap();
    groups.values().filter(|&&v| v == max_size).count() as i32
  }
}