Problem p03081 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
#define FOR(i, begin, end) for (int i = (begin); i < (end); i++)
#define REP(i, n) FOR(i, 0, n)
#define IFOR(i, begin, end) for (int i = (end)-1; i >= (begin); i--)
#define IREP(i, n) IFOR(i, 0, n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(), v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x)                                                      \
  distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x)                                                      \
  distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL << (n))
#define bit_exist(x, n) ((x >> n) & 1)
#define Ans(f, y, n)                                                           \
  if (f)                                                                       \
    cout << y << endl;                                                         \
  else                                                                         \
    cout << n << endl;
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v)                                                              \
  cout << #v << "=" << endl;                                                   \
  REP(i, v.size()) { cout << v[i] << ","; }                                    \
  cout << endl;
#define mdebug(m)                                                              \
  cout << #m << "=" << endl;                                                   \
  REP(i, m.size()) {                                                           \
    REP(j, m[i].size()) { cout << m[i][j] << ","; }                            \
    cout << endl;                                                              \
  }
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

template <typename T> void readv(vector<T> &a) { REP(i, a.size()) cin >> a[i]; }
void readv_m1(vector<int> &a) {
  REP(i, a.size()) {
    cin >> a[i];
    a[i]--;
  }
}

// int dx[4] = {1,0,-1,0};
// int dy[4] = {0,1,0,-1};

int mod = 1000000007;
// int mod = 998244353;
#define Add(x, y) x = (x + (y)) % mod
#define Mult(x, y) x = (x * (y)) % mod

signed main() {

  int N, Q;
  cin >> N >> Q;
  string s;
  cin >> s;
  vector<char> t(Q), d(Q);
  vec D(N);
  REP(i, Q) {
    cin >> t[i] >> d[i];
    if (d[i] == 'L')
      D[i] = -1;
    else
      D[i] = 1;
  }

  int i1 = -1, i2 = N;
  while (i2 - i1 > 1) {
    int i0 = (i1 + i2) / 2;
    int now = i0;
    bool fall = false;
    REP(i, Q) if (t[i] == s[now]) {
      now += D[i];
      if (now >= N)
        break;
      if (now < 0) {
        fall = true;
        break;
      }
    }
    if (fall)
      i1 = i0;
    else
      i2 = i0;
  }

  int j1 = -1, j2 = N;
  while (j2 - j1 > 1) {
    int j0 = (j1 + j2) / 2;
    int now = j0;
    bool fall = false;
    REP(i, Q) if (t[i] == s[now]) {
      now += D[i];
      if (now < 0)
        break;
      if (now >= N) {
        fall = true;
        break;
      }
    }
    if (fall)
      j2 = j0;
    else
      j1 = j0;
  }
  // debug(i1);
  // debug(j1);
  int ans = j1 - i1;
  cout << ans << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
struct fast_ios {
  fast_ios() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  };
} fast_ios_;
#define FOR(i, begin, end) for (int i = (begin); i < (end); i++)
#define REP(i, n) FOR(i, 0, n)
#define IFOR(i, begin, end) for (int i = (end)-1; i >= (begin); i--)
#define IREP(i, n) IFOR(i, 0, n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(), v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x)                                                      \
  distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x)                                                      \
  distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL << (n))
#define bit_exist(x, n) ((x >> n) & 1)
#define Ans(f, y, n)                                                           \
  if (f)                                                                       \
    cout << y << endl;                                                         \
  else                                                                         \
    cout << n << endl;
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v)                                                              \
  cout << #v << "=" << endl;                                                   \
  REP(i, v.size()) { cout << v[i] << ","; }                                    \
  cout << endl;
#define mdebug(m)                                                              \
  cout << #m << "=" << endl;                                                   \
  REP(i, m.size()) {                                                           \
    REP(j, m[i].size()) { cout << m[i][j] << ","; }                            \
    cout << endl;                                                              \
  }
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

template <typename T> void readv(vector<T> &a) { REP(i, a.size()) cin >> a[i]; }
void readv_m1(vector<int> &a) {
  REP(i, a.size()) {
    cin >> a[i];
    a[i]--;
  }
}

// int dx[4] = {1,0,-1,0};
// int dy[4] = {0,1,0,-1};

int mod = 1000000007;
// int mod = 998244353;
#define Add(x, y) x = (x + (y)) % mod
#define Mult(x, y) x = (x * (y)) % mod

signed main() {

  int N, Q;
  cin >> N >> Q;
  string s;
  cin >> s;
  vector<char> t(Q), d(Q);
  vec D(Q);
  REP(i, Q) {
    cin >> t[i] >> d[i];
    if (d[i] == 'L')
      D[i] = -1;
    else
      D[i] = 1;
  }

  int i1 = -1, i2 = N;
  while (i2 - i1 > 1) {
    int i0 = (i1 + i2) / 2;
    int now = i0;
    bool fall = false;
    REP(i, Q) if (t[i] == s[now]) {
      now += D[i];
      if (now >= N)
        break;
      if (now < 0) {
        fall = true;
        break;
      }
    }
    if (fall)
      i1 = i0;
    else
      i2 = i0;
  }

  int j1 = -1, j2 = N;
  while (j2 - j1 > 1) {
    int j0 = (j1 + j2) / 2;
    int now = j0;
    bool fall = false;
    REP(i, Q) if (t[i] == s[now]) {
      now += D[i];
      if (now < 0)
        break;
      if (now >= N) {
        fall = true;
        break;
      }
    }
    if (fall)
      j2 = j0;
    else
      j1 = j0;
  }
  // debug(i1);
  // debug(j1);
  int ans = j1 - i1;
  cout << ans << endl;

  return 0;
}