LeetCode 0316 - Remove Duplicate Letters

Input: "bcabc"
Output: "abc"

Input: "cbacdcbc"
Output: "acdb"

class Solution {
public:
    std::string removeDuplicateLetters(std::string s) {
        auto visit = std::map<char, bool>();
        auto count = std::map<char, int>();

        for(auto c : s) {
            count[c]++;
        }

        auto res = std::string();
        for(auto c : s) {
            count[c]--;

            if(visit[c]) {
                continue;
            }

            while(!res.empty() && c < res.back() && count[res.back()] != 0) {
                visit[res.back()] = false;
                res.pop_back();
            }

            res += c;
            visit[c] = true;
        }

        return res;
    }
};