/*
 * @lc app=leetcode id=402 lang=java
 *
 * [402] Remove K Digits
 */

// @lc code=start
class Solution {
    public String removeKdigits(String num, int k) {
        ArrayDeque<Character> stack = new ArrayDeque<>();
        for(int i = 0; i < num.length(); i++) {
            if(k == 0) {
                stack.addLast(num.charAt(i));
            } else {
                while(k>0 && !stack.isEmpty() && stack.peekLast() > num.charAt(i)) {
                    k--;
                    stack.removeLast();
                }
                stack.addLast(num.charAt(i));
            }
        }
        while(!stack.isEmpty() && k > 0) {
            k--;
            stack.removeLast();
        }
        // 消除开头的0
        while(!stack.isEmpty() && stack.getFirst()=='0') {
            stack.removeFirst();
        }
        StringBuilder sb = new StringBuilder();   
        for(Character c : stack) {
            sb.append(c);
        }
        return sb.isEmpty()?"0":sb.toString();
    }
}
// @lc code=end