Problem p03261 (C++)

Buggy submission (status: Runtime Error):
// ↓template↓

#include "bits/stdc++.h"
using namespace std;
#define Would
#define you
#define all(n) n.begin(), n.end()
#define rall(n) n.rbegin(), n.rend()
typedef long long ll;
const ll INF = 1e18;
const ll MOD = 1e9 + 7;
const double pi = acos(-1);
const ll SIZE = 1 << 17;
int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1}, alp[30];
ll fac[200005], finv[200005], inv[200005];
vector<ll> dij;
struct edge {
  ll to, cost;
};
vector<vector<edge>> G;

ll mod_pow(ll a, ll b) {
  ll res = 1, mul = a;
  for (int i = 0; i < 31; ++i) {
    if (b >> i & 1) {
      res *= mul;
      res %= MOD;
    }
    mul = (mul * mul) % MOD;
  }
  return res;
}

void addedge(int from, int to, int cost) {
  G[from].push_back({to, cost});
  G[to].push_back({from, cost});
}

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

template <typename T> void outp(vector<T> v) {
  for (int i = 0; i < v.size(); ++i) {
    cout << v[i];
    if (i != v.size() - 1) {
      cout << " ";
    }
  }
}

// ↑template↑

int main() {

  int n;
  cin >> n;
  vector<string> v;
  bool ok = 1;
  for (int i = 0; i < n; ++i) {
    cin >> v[i];
    if (i != 0) {
      if (v[i][0] != v[i - 1][v[i - 1].length() - 1]) {
        ok = 0;
      }
    }
  }

  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      if (i != j && v[i] == v[j]) {
        ok = 0;
      }
    }
  }

  if (ok) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }
}

Fixed submission:
// ↓template↓

#include "bits/stdc++.h"
using namespace std;
#define Would
#define you
#define all(n) n.begin(), n.end()
#define rall(n) n.rbegin(), n.rend()
typedef long long ll;
const ll INF = 1e18;
const ll MOD = 1e9 + 7;
const double pi = acos(-1);
const ll SIZE = 1 << 17;
int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1}, alp[30];
ll fac[200005], finv[200005], inv[200005];
vector<ll> dij;
struct edge {
  ll to, cost;
};
vector<vector<edge>> G;

ll mod_pow(ll a, ll b) {
  ll res = 1, mul = a;
  for (int i = 0; i < 31; ++i) {
    if (b >> i & 1) {
      res *= mul;
      res %= MOD;
    }
    mul = (mul * mul) % MOD;
  }
  return res;
}

void addedge(int from, int to, int cost) {
  G[from].push_back({to, cost});
  G[to].push_back({from, cost});
}

template <typename T> vector<T> make_v(size_t a) { return vector<T>(a); }

template <typename T, typename... Ts> auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t)
    fill_v(e, v);
}

template <typename T> void outp(vector<T> v) {
  for (int i = 0; i < v.size(); ++i) {
    cout << v[i];
    if (i != v.size() - 1) {
      cout << " ";
    }
  }
}

// ↑template↑

int main() {

  int n;
  cin >> n;
  vector<string> v(n);
  bool ok = 1;
  for (int i = 0; i < n; ++i) {
    cin >> v[i];
    if (i != 0) {
      if (v[i][0] != v[i - 1][v[i - 1].length() - 1]) {
        ok = 0;
      }
    }
  }

  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      if (i != j && v[i] == v[j]) {
        ok = 0;
      }
    }
  }

  if (ok) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }
}