Problem p02692 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <iostream>
#include <stdio.h>
using namespace std;
#include <vector>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define INF 1e9
#define llINF 1e18
#define MOD 1000000007
#define pb push_back
#define ll long long
#define ull unsigned long long
#define vint vector<int>
#define vll vector<ll>

// #include <stack>
// #include <queue>

/*
#include <math.h>
int standerd = int(pow(10.0,9.0)) + 7;
*/
string ans_Yes = "Yes";
string ans_No = "No";
string ans_yes = "yes";
string ans_no = "no";

ll ABC[4];
ll N;
ll A;
ll B;
ll C;

int main() {

  cin >> N;
  cin >> A;
  cin >> B;
  cin >> C;
  ABC[0] = A;
  ABC[1] = B;
  ABC[2] = C;
  string VAL[4];
  VAL[0] = "A";
  VAL[1] = "B";
  VAL[2] = "C";

  string flag = "Yes";
  vector<string> ans;
  vector<string> ss;
  string sentaku;
  int index1;
  int index2;
  if (A == 0 && B == 0 && C == 0) {
    cout << "No" << endl;
  } else {
    rep(ni, N) {
      cin >> sentaku;
      ss.push_back(sentaku);
    }
    rep(ni, N) {
      sentaku = ss[ni];
      if (sentaku == "AB") {
        index1 = 0;
        index2 = 1;
      } else if (sentaku == "BC") {
        index1 = 1;
        index2 = 2;
      } else {
        index1 = 0;
        index2 = 2;
      }

      if (ABC[index1] == 0 && ABC[index2] == 0) {
        flag = "No";
        break;
      } else if (ABC[index1] == 1 && ABC[index2] == 1 && ni + 1 < N) {
        if (ss[ni + 1].substr(0, 1) == VAL[index1]) {
          if (ss[ni + 1].substr(1, 1) == VAL[index2]) {
            ABC[index1]--;
            ABC[index2]++;
            ans.push_back(VAL[index2]);
          } else {
            ABC[index2]--;
            ABC[index1]++;
            ans.push_back(VAL[index1]);
          }
        } else if (ss[ni + 1].substr(1, 1) == VAL[index1]) {
          ABC[index2]--;
          ABC[index1]++;
          ans.push_back(VAL[index1]);
        } else {
          ABC[index1]--;
          ABC[index2]++;
          ans.push_back(VAL[index2]);
        }
      } else {
        if (ABC[index1] >= ABC[index2]) {
          ABC[index1]--;
          ABC[index2]++;
          ans.push_back(VAL[index2]);
        } else {
          ABC[index2]--;
          ABC[index1]++;
          ans.push_back(VAL[index1]);
        }
      }
    }
    cout << flag << endl;
    if (flag == "Yes") {
      rep(ni, ans.size()) { cout << ans[ni] << endl; }
    }
  }
}

Error: 0

Fixed submission:

#include <algorithm>
#include <iostream>
#include <stdio.h>
using namespace std;
#include <vector>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define INF 1e9
#define llINF 1e18
#define MOD 1000000007
#define pb push_back
#define ll long long
#define ull unsigned long long
#define vint vector<int>
#define vll vector<ll>

// #include <stack>
// #include <queue>

/*
#include <math.h>
int standerd = int(pow(10.0,9.0)) + 7;
*/
string ans_Yes = "Yes";
string ans_No = "No";
string ans_yes = "yes";
string ans_no = "no";

ll ABC[4];
ll N;
ll A;
ll B;
ll C;

int main() {

  cin >> N;
  cin >> A;
  cin >> B;
  cin >> C;
  ABC[0] = A;
  ABC[1] = B;
  ABC[2] = C;
  string VAL[4];
  VAL[0] = "A";
  VAL[1] = "B";
  VAL[2] = "C";

  string flag = "Yes";
  vector<string> ans;
  vector<string> ss;
  string sentaku;
  int index1;
  int index2;
  if (A == 0 && B == 0 && C == 0) {
    cout << "No" << endl;
  } else {
    rep(ni, N) {
      cin >> sentaku;
      ss.push_back(sentaku);
    }
    rep(ni, N) {
      sentaku = ss[ni];
      if (sentaku == "AB") {
        index1 = 0;
        index2 = 1;
      } else if (sentaku == "BC") {
        index1 = 1;
        index2 = 2;
      } else {
        index1 = 0;
        index2 = 2;
      }

      if (ABC[index1] == 0 && ABC[index2] == 0) {
        flag = "No";
        break;
      } else if (ABC[index1] == 1 && ABC[index2] == 1) {
        if (ss[ni + 1].substr(0, 1) == VAL[index1] &&
            ss[ni + 1].substr(1, 1) == VAL[index2]) {
          ABC[index1]--;
          ABC[index2]++;
          ans.push_back(VAL[index2]);
        } else if (ss[ni + 1].substr(0, 1) == VAL[index1]) {
          ABC[index2]--;
          ABC[index1]++;
          ans.push_back(VAL[index1]);
        } else if (ss[ni + 1].substr(1, 1) == VAL[index1]) {
          ABC[index2]--;
          ABC[index1]++;
          ans.push_back(VAL[index1]);
        } else {
          ABC[index1]--;
          ABC[index2]++;
          ans.push_back(VAL[index2]);
        }
      } else {
        if (ABC[index1] >= ABC[index2]) {
          ABC[index1]--;
          ABC[index2]++;
          ans.push_back(VAL[index2]);
        } else {
          ABC[index2]--;
          ABC[index1]++;
          ans.push_back(VAL[index1]);
        }
      }
    }
    cout << flag << endl;
    if (flag == "Yes") {
      rep(ni, ans.size()) { cout << ans[ni] << endl; }
    }
  }
}