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