1047. Remove All Adjacent Duplicates In String

O(n) time O(n) space
这道题的意思是一定要『一对』才remove

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