316. 去除重复字母

316. 去除重复字母

class Solution {
    public String removeDuplicateLetters(String s) {
        int len = s.length();
        if (len == 0 || s.length() == 1)
            return s;
        Map<Character, Integer> map = new HashMap<>();
        Map<Character, Boolean> exit = new HashMap<>();
        for (int i = 0; i < len; i++) {
            Character c = s.charAt(i);
            map.put(c, map.getOrDefault(c, 0) + 1);
            exit.put(c, false);
        }
        Deque<Character> deque = new LinkedList<>();
        for (int i = 0; i < len; i++) {
            Character c = s.charAt(i);
            while (!deque.isEmpty() && exit.get(c) == false && deque.peekLast() > c && map.get(deque.peekLast()) > 1) {
                Character temp = deque.pollLast();
                map.put(temp, map.get(temp) - 1);
                exit.put(temp, false);
            }
            if (exit.get(c) == false) {
                deque.offerLast(c);
                exit.put(c, true);
            } else {
                map.put(c, map.get(c) - 1);
            }
        }
        StringBuilder ans = new StringBuilder();
        while (!deque.isEmpty()) {
            ans.append(deque.pollFirst());
        }
        return ans.toString();
    }
}