// Time Complexity: O(n), where n is s.size() + t.size()
// Space Complexity: O(1)
class Solution {
public:
    char findTheDifference(string s, string t) {
        int k = 0;

        assert(t.size() - s.size() == 1);

        for (auto c : t) {
            k ^= 1 << (c - 'a');
        }
        for (auto c : s) {
            k ^= 1 << (c - 'a');
        }
        for (int i = 1; i <= 26; i++) {
            if (k == (1 << i)) {
                return i + 'a';
            }
        }
        return 'a';
    }
};