LeetCode 1047. Remove All Adjacent Duplicates In String

class Solution {
public:
    string removeDuplicates(string S) {
        string stk;
        for (char c : S)
            if (!stk.empty() && stk.back() == c)
                stk.pop_back();
            else
                stk.push_back(c);
        return stk;
    }
};