#include <string>
#include <iostream>
#include <vector>
#include <unordered_map>

using namespace std;

string solution(vector<string> participants, vector<string> completions) {
    string answer = "";
    
    unordered_map<string, int> hash;
    
    for (auto participant : participants) {
        hash[participant] += 1;
    }
    
    for (auto completion : completions) {
        hash[completion] -= 1;
    }
    
    for (auto participant : participants) {
        if (hash[participant] == 1)
            answer = participant;
    }
    
    return answer;
}

다른풀이

#include <string>
#include <vector>
#include <algorithm>

using namespace std;

string solution(vector<string> participant, vector<string> completion) {
    string answer = "";
    sort(participant.begin(), participant.end());
    sort(completion.begin(), completion.end());
    for(int i=0; i<completion.size(); i++)
    {
        if(participant[i] != completion[i])
            return participant[i];
    }
    
    return participant[participant.size() - 1];
    //return answer;
}