LeetCode 402

class Solution {
    public String removeKdigits(String num, int k) {
        k = Math.min(num.length(), k);
        StringBuilder sb = new StringBuilder();
        for (char c: num.toCharArray()) {
            while (k > 0 && sb.length() > 0 && sb.charAt(sb.length () - 1) > c) {
                sb.deleteCharAt(sb.length() - 1);
                k--;
            }
            sb.append(c);
        }
        while (k-- > 0) sb.deleteCharAt(sb.length() - 1);
        k = 0;
        int cnt = sb.length();
        while (k < cnt && sb.charAt(k) == '0') k++;
        sb.delete(0, k);
        if (k == cnt) sb.append("0");
        return sb.toString();
    }
}