RemoveKDigits.java

package com.smlnskgmail.jaman.leetcodejava.medium;

import java.util.Stack;

// https://leetcode.com/problems/remove-k-digits/
public class RemoveKDigits {

    private final String num;
    private int k;

    public RemoveKDigits(String num, int k) {
        this.num = num;
        this.k = k;
    }

    public String solution() {
        if (num.length() == k) {
            return "0";
        }
        Stack<Character> stack = new Stack<>();
        int i = 0;
        while (i < num.length()) {
            while (k > 0 && !stack.isEmpty() && stack.peek() > num.charAt(i)) {
                stack.pop();
                k--;
            }
            stack.push(num.charAt(i));
            i++;
        }
        while (k > 0) {
            stack.pop();
            k--;
        }
        var result = new StringBuilder();
        while (!stack.isEmpty()) {
            result.append(stack.pop());
        }
        result.reverse();
        while (result.length() > 1 && result.charAt(0) == '0') {
            result.deleteCharAt(0);
        }
        return result.toString();
    }

}