题目：

题解：

class Solution {
public:string removeDuplicateLetters(string s) {vector<int> vis(26), num(26);for (char ch : s) {num[ch - 'a']++;}string stk;for (char ch : s) {if (!vis[ch - 'a']) {while (!stk.empty() && stk.back() > ch) {if (num[stk.back() - 'a'] > 0) {vis[stk.back() - 'a'] = 0;stk.pop_back();} else {break;}}vis[ch - 'a'] = 1;stk.push_back(ch);}num[ch - 'a'] -= 1;}return stk;}
};