题目：
题解：
class Solution {public String removeDuplicateLetters(String s) {boolean[] vis = new boolean[26];int[] num = new int[26];for (int i = 0; i < s.length(); i++) {num[s.charAt(i) - 'a']++;}StringBuffer sb = new StringBuffer();for (int i = 0; i < s.length(); i++) {char ch = s.charAt(i);if (!vis[ch - 'a']) {while (sb.length() > 0 && sb.charAt(sb.length() - 1) > ch) {if (num[sb.charAt(sb.length() - 1) - 'a'] > 0) {vis[sb.charAt(sb.length() - 1) - 'a'] = false;sb.deleteCharAt(sb.length() - 1);} else {break;}}vis[ch - 'a'] = true;sb.append(ch);}num[ch - 'a'] -= 1;}return sb.toString();}
}
