Remove k digits

String Stack Greedy Monotonic Stack

36.4% acceptance

Jan 13, 2026

10530

546

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

Solution

Rust

Time O(n²)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn remove_kdigits(num: String, k: i32) -> String {
    let mut k = k as usize;
    let mut stack: Vec<char> = Vec::new();
    
    for ch in num.chars() {
      while k > 0 && !stack.is_empty() && stack.last().unwrap() > &ch {
        stack.pop();
        k -= 1;
      }
      stack.push(ch);
    }
    
    while k > 0 {
      stack.pop();
      k -= 1;
    }
    
    let result: String = stack.into_iter()
      .skip_while(|&c| c == '0')
      .collect();
    
    if result.is_empty() {
      "0".to_string()
    } else {
      result
    }
  }
}