045b.cpp

#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
//using mint = modint1000000007;
// using mint = modint;  /*このときmint::set_mod(mod)のようにしてmodを底にする*/
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repi(i,a,b) for(int i = a; i <= (int)(b); i++)
#define rng(i,a,b) for(int i = a; i < (int)(b); i++)
#define rrng(i,a,b) for(int i = a; i >= (int)(b); i--)
#define pb push_back
#define eb emplace_back
#define pob pop_back
#define si(a) (int)a.size()
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define ret(x) { cout<<(x)<<endl;}
typedef long long ll;
using namespace std;
using P = pair<ll,ll>;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
ll mul(ll a, ll b) { if (a == 0) return 0; if (LINF / a < b) return LINF; return min(LINF, a * b); }
ll mod(ll x, ll y){return (x % y + y) % y;}
char itoc(int x){return x + '0';}
int ctoi(char c){return c - '0';}
void chmin(ll& x,ll y){x = min(x,y);}
void chmax(ll& x,ll y){x = max(x,y);}
ll getnum(ll x, ll y, ll H, ll W) { (void) H; return (x * W + y);}
template<typename T>
void print(vector<T> &p){rep(i,si(p)) cout << p[i] << " "; cout << endl;}
ll ceil(ll x , ll y){return (x+y-1)/y;}

int main(){

    string SA,SB,SC;
    cin >> SA >> SB >> SC;

    deque<char> sa,sb,sc;
    rep(i,si(SA)){
        sa.push_back(SA[i]);
    }
    rep(i,si(SB)){
        sb.push_back(SB[i]);
    }
    rep(i,si(SC)){
        sc.push_back(SC[i]);
    }

    char now = 'a';
    
    while(!sa.empty() || !sb.empty() || !sc.empty()){
        if(now == 'a'){
            if(sa.empty()){
                cout << 'A' << endl;
                return 0;
            }
            now = sa.front();
            sa.pop_front();
        }
        else if(now == 'b'){
            if(sb.empty()){
                cout << 'B' << endl;
                return 0;
            }
            now = sb.front();
            sb.pop_front();
        }
        else{
            if(sc.empty()){
                cout << 'C' << endl;
                return 0;
            }
            now = sc.front();
            sc.pop_front();
        }
    }

    return 0;
}