402. 移掉K位数字

402. 移掉K位数字

class Solution {
    public String removeKdigits(String num, int k) {        
        Deque<Character> digits = new LinkedList<>();
        for (int i = 0; i < num.length(); i++) {
            Character digit = num.charAt(i);
            while (!digits.isEmpty() && k > 0 && digits.peekLast() > digit) {
                digits.pollLast();
                k--;
            }
            digits.offerLast(digit);
        }
        for (int i = 0; i < k; i++) {
            digits.pollLast();
        }
        StringBuilder ans = new StringBuilder();
        boolean leadingZero = true;
        while (!digits.isEmpty()) {
            Character digit = digits.pollFirst();
            if (leadingZero && digit == '0') 
                continue;
            leadingZero = false;
            ans.append(digit);
        }
        return ans.length() == 0 ? "0" : ans.toString();
    }
}