316. Remove Duplicate Letters

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

Example:

Given

"bcabc"

Return

"abc"

Given

"cbacdcbc"

Return

"acdb"

Credits:

Special thanks to
@dietpepsi for adding this problem and creating all test cases.

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题目的意思是删除重复的元素，同时若有重复的，尽量把小的排在前面。

采用两个hash表记录，一个记录个数，另一个记录是否被访问。遍历源string，记录hash表中对应char项减一，若被访问了说明，已经在结果中处于正确位置，若没有被访问过，则检查结果string的最后一个char是不是大于当前char，且个数记录表中不为0（说明，还存在这样的char在当前char的后面），则循环检查结果string的最后一个单词，直到满足规定。结果string 初始化时候为“0”。

代码如下：

class Solution {

public:        

    string removeDuplicateLetters(string s) {

        string res="0";

        int m1[256]={0};

        int visited[256]={0};

        for(auto p:s) m1[p]++;

        for(auto p:s)

        {

            

            --m1[p];

            if(visited[p]) continue;

            while(m1[res.back()]&&p<res.back())

            {

                visited[res.back()]=0;

                res.pop_back();

            }

            visited[p]=1;

            res+=p;

        }

        return res.substr(1);

    }

};