402. Remove K Digits

/*
Time Complexity: O(N)
Space Complexity: O(N)

N = Length of the num string.
*/
class Solution {
    public String removeKdigits(String num, int k) throws IllegalArgumentException {
        if (num == null) {
            throw new IllegalArgumentException("Input num string is null");
        }
        if (num.length() == 0 || num.length() <= k) {
            return "0";
        }
        if (k == 0) {
            return num;
        }

        StringBuilder sb = new StringBuilder();
        for (char c : num.toCharArray()) {
            while (k > 0 && sb.length() != 0 && sb.charAt(sb.length()-1) > c) {
                sb.setLength(sb.length()-1);
                k--;
            }
            if (sb.length() == 1 && sb.charAt(0) == '0') {
                sb.setCharAt(0, c);
            } else {
                sb.append(c);
            }
        }

        if (k > 0) {
            sb.setLength(sb.length() - k);
        }

        return sb.toString();
    }
}