License key formatting

String

45.7% acceptance

Jan 13, 2026

1197

1463

You are given a license key represented as a string s that consists of only alphanumeric characters and dashes. The string is separated into n + 1 groups by n dashes. You are also given an integer k. We want to reformat the string s such that each group contains exactly k characters, except for the first group, which could be shorter than k but still must contain at least one character. Furthermore, there must be a dash inserted between two groups, and you should convert all lowercase letters to uppercase. Return the reformatted license key.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn license_key_formatting(s: String, k: i32) -> String {
    let chars: String = s.chars().filter(|&c| c != '-').map(|c| c.to_ascii_uppercase()).collect();
    let len = chars.len();
    if len == 0 { return String::new(); }
    
    let first_group_size = len % k as usize;
    let mut result = String::new();
    
    if first_group_size > 0 {
      result.push_str(&chars[..first_group_size]);
      if first_group_size < len {
        result.push('-');
      }
    }
    
    let mut i = first_group_size;
    while i < len {
      result.push_str(&chars[i..i + k as usize]);
      i += k as usize;
      if i < len {
        result.push('-');
      }
    }
    
    result
  }
}