ABC_45 - B - 3人でカードゲームイージー / Card Game for Three

#include <bits/stdc++.h>

#define SYNC ios::sync_with_stdio(0);
#define F first
#define S second
#define endl '\n'


using namespace std;

using ll = long long int;
using ii = pair<int, int>;
using vii = vector<ii>;
using vi = vector<int>;
using graph = vector<vi>;
const int INF = 0x3f3f3f3f;
const int MAXN = 58;
const ll mod = 1000000007;



int main() {
    SYNC
    string deck[3];
    cin >> deck[0] >> deck[1] >> deck[2];
    //cout << deck[0] << ' ' << deck[1] << ' ' << deck[2] << ' ' << endl;
    char i = deck[0][0];
    deck[0].erase(0, 1);
    //cout << deck[0][0] << endl;

    while (!deck[i-'a'].empty()) {
        int idx = i-'a';
        i = deck[i-'a'][0];
        deck[idx].erase(0, 1);
    }
    char ans = (char) toupper(i);
    cout << ans << endl;

    return 0;
}