Note:

1. Find a increasing digits number. It's kind of longest increasing subsequence but with fixed size.

2. Remember to remove the zeros from beginning.

class Solution {    public String removeKdigits(String num, int k) {        if (num.length() == 0) return "0";        int i = 1;        Stack
     
       stack = new Stack<>();        stack.push(num.charAt(0));        while (i < num.length()) {            while (k > 0 && !stack.isEmpty() && num.charAt(i) < stack.peek()) {                k--;                stack.pop();            }            stack.push(num.charAt(i++));        }                while (k > 0 && !stack.isEmpty()) {            stack.pop();            k--;        }        StringBuilder result = new StringBuilder();        while (!stack.isEmpty()) {            result.insert(0, stack.pop());        }        while (result.length() > 1 && result.charAt(0) == '0') result.deleteCharAt(0);        return result.length() == 0 ? "0" : result.toString();    }}