316. Remove Duplicate Letters

     /*
      * 316. Remove Duplicate Letters
      * 2016-7-5 by Mingyang
      * 这个题目有一定的难度，所以需要自己再研究一下
      * 这里用到了stack，首先没到一个数的时候，计数器减一，然后跟stack里面的比较
      * 如果比stack peek的还小，这个就是放到stack里面的前面的，否则就是放到后面了
      * 不过注意！！！stack在弹出的时候必须确保有备胎，也就是弹出以后必须有后补才弹
      * 不然不要弹！
      * 每次弹出来以后需要把visited设为false，每次进去以后呢也需要把visited设为true！！
      */
     public String removeDuplicateLetters(String s) {
            Stack<Character> stack = new Stack<Character>();
            // appearance count
            int[] count = new int[26];
            // used or not;
            boolean[] added = new boolean[26];
            // count appearances
            for (char ch : s.toCharArray())
                count[ch - 'a']++;
            // go through each char
            for (char ch : s.toCharArray()) {
                count[ch - 'a']--;
                if (added[ch - 'a'])
                    continue;
                // poping out the char which is bigger and still has some left in behind
                while (!stack.isEmpty() && stack.peek() > ch
                        && count[stack.peek() - 'a'] > 0)
                    added[stack.pop() - 'a'] = false;
               // add current one
                stack.push(ch);
                added[ch - 'a'] = true;
            }
               // move from stack to string
            StringBuilder sb = new StringBuilder();
            while (!stack.isEmpty()) {
                sb.append(stack.pop());
            }
            return sb.reverse().toString();
        }