一、题目

Given a string s, remove duplicate letters so that every letter appears once and only once. You must make sure your result is
the smallest in lexicographical order
among all possible results.

Example 1:

Input: s = “bcabc”
Output: “abc”
Example 2:

Input: s = “cbacdcbc”
Output: “acdb”

Constraints:

1 <= s.length <= 104
s consists of lowercase English letters.

二、题解

class Solution {
public:string removeDuplicateLetters(string s) {int n = s.size();unordered_map<char,int> map;unordered_map<char,int> inStack;stack<char> st;for(auto c:s) map[c]++;for(auto c:s){if(inStack[c] == 0){//大压小的栈while(!st.empty() && c < st.top() && map[st.top()] > 0){inStack[st.top()] = 0;st.pop();}st.push(c);inStack[c] = 1;}map[c]--;}string res = "";while(!st.empty()){res += st.top();st.pop();}reverse(res.begin(),res.end());return res;}
};