Maximum number of balls in a box

Hash Table Math Counting

74.7% acceptance

Feb 25, 2026

653

170

You are working in a ball factory where you have n balls numbered from lowLimit up to highLimit inclusive (i.e., n == highLimit - lowLimit + 1), and an infinite number of boxes numbered from 1 to infinity. Your job at this factory is to put each ball in the box with a number equal to the sum of digits of the ball's number. Given two integers lowLimit and highLimit, return the number of balls in the box with the most balls.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn count_balls(low_limit: i32, high_limit: i32) -> i32 {
    let digit_sum = |mut n: i32| -> usize {
      let mut s = 0;
      while n > 0 { s += (n % 10) as usize; n /= 10; }
      s
    };
    // Max digit sum for any number <= 10^5 is 9*5 = 45
    let mut boxes = vec![0i32; 46];
    for n in low_limit..=high_limit {
      boxes[digit_sum(n)] += 1;
    }
    *boxes.iter().max().unwrap()
  }
}