public String removeKdigits(String num, int k) {

Deque<Character> stack = new ArrayDeque<>();

for (int i = 0; i < num.length(); i++) {

while (!stack.isEmpty() && k > 0 && (stack.peekLast() - '0') > (num.charAt(i) - '0')) {

stack.pollLast();

k--;

}

stack.addLast(num.charAt(i));

}

while (k > 0) {

stack.pollLast();

k--;

}

StringBuilder ret = new StringBuilder();

boolean leadingZero = true;

while (!stack.isEmpty()) {

char digit = stack.pollFirst();

if (leadingZero && digit == '0') {

continue;

}

leadingZero = false;

ret.append(digit);

}

return ret.length() == 0 ? "0" : ret.toString();

}