Problem p03003 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <cstring>
#include <ctype.h>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#include <deque>
#define pi 3.141592653589793238
#include <chrono>

#define MOD 1000000007
#define INF 999999999999999999
#define pb push_back
#define ff first
#define ss second
#define mt make_tuple
#define ll long long
#define fast                                                                   \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL);
#define f0 get<0>
#define f1 get<1>
#define f2 get<2>

using namespace std;
ll n;
ll ncr(ll n, ll r) {
  ll i, ans = 1, k = 0;
  if (r == 0)
    return 1;
  else if (n < r)
    return 0;
  for (i = n; i >= n - r + 1; i--) {
    ans *= i;
    ans /= n - i + 1;
  }

  return ans;
}

void max_heapify(int a[100], int i) {
  int l, r, largest = i;
  l = 2 * i + 1;
  r = 2 * i + 2;
  if (l < n && a[l] > a[i])
    largest = l;
  if (r < n && a[r] > a[l])
    largest = r;
  if (largest != i) {
    int k;
    k = a[i];
    a[i] = a[largest];
    a[largest] = k;
    max_heapify(a, largest);
  }
}
int power(ll x, ll y, ll p) {
  int res = 1;
  x = x % p;

  while (y > 0) {

    if (y & 1)
      res = (res * x) % p;

    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}

bool square(ll n) {
  double x;
  x = sqrt(n);
  if (x == floor(x))
    return true;
  else
    return false;
}

bool prime(ll n) {
  ll i;
  for (i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}

ll modi(ll a, ll m) { return power(a, m - 2, m); }
void kmp(string s, ll p[]) {
  int n = (int)s.length();
  p[0] = 0;
  for (int i = 1; i < n; i++) {
    int j = p[i - 1];
    while (j > 0 && s[i] != s[j])
      j = p[j - 1];
    if (s[i] == s[j])
      j++;
    p[i] = j;
  }
}
void zfunc(string s, ll z[]) {

  ll n = (ll)s.length();
  z[0] = 0;
  for (ll i = 1, l = 0, r = 0; i < n; ++i) {
    if (i <= r)
      z[i] = min(r - i + 1, z[i - l]);
    while (i + z[i] < n && s[z[i]] == s[i + z[i]])
      ++z[i];
    if (i + z[i] - 1 > r)
      l = i, r = i + z[i] - 1;
  }
}
struct pair_hash {
  size_t operator()(const pair<int, int> &p) const noexcept {
    return hash<ll>()((((ll)p.first) << 32) | ((ll)p.second));
  }
};

bool palin(string s) {
  ll i, n;
  n = s.length();
  for (i = 0; i <= n / 2; i++) {
    if (s[i] != s[n - i - 1]) {
      return false;
    }
  }
  return true;
}
ll sum(ll n) {
  ll i;
  i = n;
  ll sum = 0;
  while (i > 0) {
    sum += i % 10;
    i = i / 10;
  }
  return sum;
}

ll nCr(ll n, ll r, ll p, ll fac[]) {

  if (r == 0)
    return 1;
  fac[0] = 1;
  for (int i = 1; i <= n; i++)
    fac[i] = fac[i - 1] * i % p;

  return (fac[n] * modi(fac[r], p) % p * modi(fac[n - r], p) % p) % p;
}

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll mod(string num, ll a) {

  ll res = 0;

  for (ll i = 0; i < num.length(); i++)
    res = (res * 10 + (ll)num[i] - '0') % a;

  return res;
}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
ll parent[1000], sizer[1000];
ll findset(ll a) {
  if (a == parent[a]) {
    return a;
  }
  parent[a] = findset(parent[a]);
  return parent[a];
}

void unionset(ll a, ll b) {
  a = findset(a);
  b = findset(b);
  if (sizer[a] < sizer[b]) {
    swap(a, b);
  }
  sizer[b] += sizer[a];
  parent[b] = a;
}
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
struct fenwick {
  vector<ll> bit;
  ll n;
  fenwick(ll n, vector<ll> v) {
    this->n = n;
    bit.assign(n, 0);
    for (ll it = 0; it < n; it++) {
      add(it, v[it]);
    }
  }

  ll sum(ll idx) {
    if (idx < 0)
      return 0;
    ll ret = 0;
    for (; idx >= 0; idx = (idx & (idx + 1)) - 1) {
      ret += bit[idx];
    }
    return ret;
  }
  void add(ll idx, ll x) {
    for (; idx < n; idx = idx | (idx + 1)) {
      bit[idx] += x;
      bit[idx] = max(0LL, bit[idx]);
    }
  }
};
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

int main() {
  // freopen("input.txt", "r", stdin);
  // freopen("output.txt", "w", stdout);
  fast;
  ll T = 1, i, j;

  // cin >> T;
  while (T--) {
    ll n, m;
    cin >> n >> m;
    vector<ll> s(n), t(m);
    for (i = 0; i < n; i++) {
      cin >> s[i];
    }
    for (i = 0; i < m; i++) {
      cin >> t[i];
    }
    ll dp[100][100] = {0};
    ll dp2[100][100] = {0};
    ll ans = 0;

    for (i = 0; i < n; i++) {
      for (j = 0; j < m; j++) {

        dp2[i + 1][j + 1] =
            (dp2[i][j + 1] + dp2[i + 1][j] - dp2[i][j] + dp[i][j] + MOD) % MOD;
        if (s[i] != t[j]) {
          dp[i + 1][j + 1] = 0;
          continue;
        }
        dp[i + 1][j + 1] = (dp2[i + 1][j + 1] + 1) % MOD;
        ans += dp[i + 1][j + 1];
        ans %= MOD;
      }
    }
    cout << (ans + 1) % MOD;
  }

  return 0;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <cstring>
#include <ctype.h>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#include <deque>
#define pi 3.141592653589793238
#include <chrono>

#define MOD 1000000007
#define INF 999999999999999999
#define pb push_back
#define ff first
#define ss second
#define mt make_tuple
#define ll long long
#define fast                                                                   \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(NULL);                                                               \
  cout.tie(NULL);
#define f0 get<0>
#define f1 get<1>
#define f2 get<2>

using namespace std;
ll n;
ll ncr(ll n, ll r) {
  ll i, ans = 1, k = 0;
  if (r == 0)
    return 1;
  else if (n < r)
    return 0;
  for (i = n; i >= n - r + 1; i--) {
    ans *= i;
    ans /= n - i + 1;
  }

  return ans;
}

void max_heapify(int a[100], int i) {
  int l, r, largest = i;
  l = 2 * i + 1;
  r = 2 * i + 2;
  if (l < n && a[l] > a[i])
    largest = l;
  if (r < n && a[r] > a[l])
    largest = r;
  if (largest != i) {
    int k;
    k = a[i];
    a[i] = a[largest];
    a[largest] = k;
    max_heapify(a, largest);
  }
}
int power(ll x, ll y, ll p) {
  int res = 1;
  x = x % p;

  while (y > 0) {

    if (y & 1)
      res = (res * x) % p;

    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}

bool square(ll n) {
  double x;
  x = sqrt(n);
  if (x == floor(x))
    return true;
  else
    return false;
}

bool prime(ll n) {
  ll i;
  for (i = 2; i * i <= n; i++) {
    if (n % i == 0)
      return false;
  }
  return true;
}

ll modi(ll a, ll m) { return power(a, m - 2, m); }
void kmp(string s, ll p[]) {
  int n = (int)s.length();
  p[0] = 0;
  for (int i = 1; i < n; i++) {
    int j = p[i - 1];
    while (j > 0 && s[i] != s[j])
      j = p[j - 1];
    if (s[i] == s[j])
      j++;
    p[i] = j;
  }
}
void zfunc(string s, ll z[]) {

  ll n = (ll)s.length();
  z[0] = 0;
  for (ll i = 1, l = 0, r = 0; i < n; ++i) {
    if (i <= r)
      z[i] = min(r - i + 1, z[i - l]);
    while (i + z[i] < n && s[z[i]] == s[i + z[i]])
      ++z[i];
    if (i + z[i] - 1 > r)
      l = i, r = i + z[i] - 1;
  }
}
struct pair_hash {
  size_t operator()(const pair<int, int> &p) const noexcept {
    return hash<ll>()((((ll)p.first) << 32) | ((ll)p.second));
  }
};

bool palin(string s) {
  ll i, n;
  n = s.length();
  for (i = 0; i <= n / 2; i++) {
    if (s[i] != s[n - i - 1]) {
      return false;
    }
  }
  return true;
}
ll sum(ll n) {
  ll i;
  i = n;
  ll sum = 0;
  while (i > 0) {
    sum += i % 10;
    i = i / 10;
  }
  return sum;
}

ll nCr(ll n, ll r, ll p, ll fac[]) {

  if (r == 0)
    return 1;
  fac[0] = 1;
  for (int i = 1; i <= n; i++)
    fac[i] = fac[i - 1] * i % p;

  return (fac[n] * modi(fac[r], p) % p * modi(fac[n - r], p) % p) % p;
}

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
ll mod(string num, ll a) {

  ll res = 0;

  for (ll i = 0; i < num.length(); i++)
    res = (res * 10 + (ll)num[i] - '0') % a;

  return res;
}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
ll parent[1000], sizer[1000];
ll findset(ll a) {
  if (a == parent[a]) {
    return a;
  }
  parent[a] = findset(parent[a]);
  return parent[a];
}

void unionset(ll a, ll b) {
  a = findset(a);
  b = findset(b);
  if (sizer[a] < sizer[b]) {
    swap(a, b);
  }
  sizer[b] += sizer[a];
  parent[b] = a;
}
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
struct fenwick {
  vector<ll> bit;
  ll n;
  fenwick(ll n, vector<ll> v) {
    this->n = n;
    bit.assign(n, 0);
    for (ll it = 0; it < n; it++) {
      add(it, v[it]);
    }
  }

  ll sum(ll idx) {
    if (idx < 0)
      return 0;
    ll ret = 0;
    for (; idx >= 0; idx = (idx & (idx + 1)) - 1) {
      ret += bit[idx];
    }
    return ret;
  }
  void add(ll idx, ll x) {
    for (; idx < n; idx = idx | (idx + 1)) {
      bit[idx] += x;
      bit[idx] = max(0LL, bit[idx]);
    }
  }
};
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

int main() {
  // freopen("input.txt", "r", stdin);
  // freopen("output.txt", "w", stdout);
  fast;
  ll T = 1, i, j;

  // cin >> T;
  while (T--) {
    ll n, m;
    cin >> n >> m;
    vector<ll> s(n), t(m);
    for (i = 0; i < n; i++) {
      cin >> s[i];
    }
    for (i = 0; i < m; i++) {
      cin >> t[i];
    }
    ll dp[2001][2001] = {0};
    ll dp2[2001][2001] = {0};
    ll ans = 0;

    for (i = 0; i < n; i++) {
      for (j = 0; j < m; j++) {

        dp2[i + 1][j + 1] =
            (dp2[i][j + 1] + dp2[i + 1][j] - dp2[i][j] + dp[i][j] + MOD) % MOD;
        if (s[i] != t[j]) {
          dp[i + 1][j + 1] = 0;
          continue;
        }
        dp[i + 1][j + 1] = (dp2[i + 1][j + 1] + 1) % MOD;
        ans += dp[i + 1][j + 1];
        ans %= MOD;
      }
    }
    cout << (ans + 1) % MOD;
  }

  return 0;
}