Solution.java

class Solution {
    public String removeKdigits(String num, int k) {
        StringBuilder stk = new StringBuilder();
        for (char c : num.toCharArray()) {
            while (k > 0 && stk.length() > 0 && stk.charAt(stk.length() - 1) > c) {
                stk.deleteCharAt(stk.length() - 1);
                --k;
            }
            stk.append(c);
        }
        for (; k > 0; --k) {
            stk.deleteCharAt(stk.length() - 1);
        }
        int i = 0;
        for (; i < stk.length() && stk.charAt(i) == '0'; ++i) {
        }
        String ans = stk.substring(i);
        return "".equals(ans) ? "0" : ans;
    }
}