find the difference leetcode

class Solution {
public:
    // Time Complexity:- O(max(N,M))
    // Space Complexity:- O(26)
    char findTheDifference(string s, string t) {
        vector<int> dp(26);
        for(auto& c:s){
            dp[c-'a']++;
        }
        for(auto& c:t){
            if(!dp[c-'a']){
                return c;
            }
            dp[c-'a']--;
        }
        return 'a';
    }
};