Problem p00961 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define dbg(...)                                                               \\
  do {                                                                         \\
    cerr << __LINE__ << ": ";                                                  \\
    dbgprint(#__VA_ARGS__, __VA_ARGS__);                                       \\
  } while (0);

using namespace std;

namespace std {
template <class S, class T> struct hash<pair<S, T>> {
  size_t operator()(const pair<S, T> &p) const {
    return ((size_t)1e9 + 7) * hash<S>()(p.first) + hash<T>()(p.second);
  }
};
template <class T> struct hash<vector<T>> {
  size_t operator()(const vector<T> &v) const {
    size_t h = 0;
    for (auto i : v)
      h = h * ((size_t)1e9 + 7) + hash<T>()(i) + 1;
    return h;
  }
};
} // namespace std
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[ ";
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? " ]" : ", ");
  return os;
}
template <class T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "{ ";
  for (const auto &i : v)
    os << i << ", ";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const map<T, U> &v) {
  os << "{";
  for (const auto &i : v)
    os << " " << i.first << ": " << i.second << ",";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
void dbgprint(const string &fmt) { cerr << endl; }
template <class H, class... T>
void dbgprint(const string &fmt, const H &h, const T &...r) {
  cerr << fmt.substr(0, fmt.find(",")) << "= " << h << " ";
  dbgprint(fmt.substr(fmt.find(",") + 1), r...);
}
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
const int inf = (int)1e9;
const double INF = 1e12, EPS = 1e-9;

template <class T> struct SegTree {
  T *dat, *lazy;
  int n;
  SegTree(int size = 1000000) {
    for (n = 1; n < size; n *= 2)
      ;
    dat = new T[2 * n - 1];
    lazy = new T[2 * n - 1];
    init();
  }
  ~SegTree() {
    delete[] dat;
    delete[] lazy;
  }

  void init() {
    rep(i, 2 * n - 1) dat[i] = 0;
    rep(i, 2 * n - 1) lazy[i] = 0;
  }
  void add(int a, int b, ll v, int k, int l, int r) {
    if (r <= a || b <= l)
      return;
    if (a <= l && r <= b) {
      lazy[k] += v;
      return;
    }
    if (lazy[k]) {
      lazy[k * 2 + 1] += lazy[k];
      lazy[k * 2 + 2] += lazy[k];
      lazy[k] = 0;
    }
    add(a, b, v, k * 2 + 1, l, (l + r) / 2);
    add(a, b, v, k * 2 + 2, (l + r) / 2, r);
    dat[k] =
        min(dat[k * 2 + 1] + lazy[k * 2 + 1], dat[k * 2 + 2] + lazy[k * 2 + 2]);
  }
  T query(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return inf;
    if (a <= l && r <= b)
      return dat[k] + lazy[k];
    if (lazy[k]) {
      lazy[k * 2 + 1] += lazy[k];
      lazy[k * 2 + 2] += lazy[k];
      lazy[k] = 0;
    }
    T vl = query(a, b, k * 2 + 1, l, (l + r) / 2);
    T vr = query(a, b, k * 2 + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
  T query(int a, int b) { return query(a, b, 0, 0, n); }
  void add(int a, int b, ll v) { add(a, b, v, 0, 0, n); }
};
int main() {
  cin.tie(0);
  cin.sync_with_stdio(0);

  int n, len;
  cin >> n >> len;
  string s, t;
  cin >> s >> t;

  vi ans(n + 1);
  SegTree<int> seg(n + 2);
  rep(i, n) {
    int best = inf;
    if (s[i] == t[i])
      best = ans[i];
    if (i && t[i - 1] != t[i])
      seg.add(0, i, 1);
    best = min(best, (seg.query(max(0, i + 1 - len), i + 1) + 1) / 2 + 1);

    seg.add(i + 1, i + 2, 2 * best);
    ans[i + 1] = best;
  }
  cout << ans[n] << endl;
  return 0;
}

Error: TLE

Fixed submission:

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)n; i++)
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define dbg(...)                                                               \\
  do {                                                                         \\
    cerr << __LINE__ << ": ";                                                  \\
    dbgprint(#__VA_ARGS__, __VA_ARGS__);                                       \\
  } while (0);

using namespace std;

namespace std {
template <class S, class T> struct hash<pair<S, T>> {
  size_t operator()(const pair<S, T> &p) const {
    return ((size_t)1e9 + 7) * hash<S>()(p.first) + hash<T>()(p.second);
  }
};
template <class T> struct hash<vector<T>> {
  size_t operator()(const vector<T> &v) const {
    size_t h = 0;
    for (auto i : v)
      h = h * ((size_t)1e9 + 7) + hash<T>()(i) + 1;
    return h;
  }
};
} // namespace std
template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "[ ";
  rep(i, v.size()) os << v[i] << (i == v.size() - 1 ? " ]" : ", ");
  return os;
}
template <class T> ostream &operator<<(ostream &os, const set<T> &v) {
  os << "{ ";
  for (const auto &i : v)
    os << i << ", ";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const map<T, U> &v) {
  os << "{";
  for (const auto &i : v)
    os << " " << i.first << ": " << i.second << ",";
  return os << "}";
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
void dbgprint(const string &fmt) { cerr << endl; }
template <class H, class... T>
void dbgprint(const string &fmt, const H &h, const T &...r) {
  cerr << fmt.substr(0, fmt.find(",")) << "= " << h << " ";
  dbgprint(fmt.substr(fmt.find(",") + 1), r...);
}
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
const int inf = (int)1e9;
const double INF = 1e12, EPS = 1e-9;

template <class T> struct SegTree {
  T *dat, *lazy;
  int n;
  SegTree(int size = 1000000) {
    for (n = 1; n < size; n *= 2)
      ;
    dat = new T[2 * n - 1];
    lazy = new T[2 * n - 1];
    init();
  }
  ~SegTree() {
    delete[] dat;
    delete[] lazy;
  }

  void init() {
    rep(i, 2 * n - 1) dat[i] = 0;
    rep(i, 2 * n - 1) lazy[i] = 0;
  }
  void add(int a, int b, ll v, int k, int l, int r) {
    if (r <= a || b <= l)
      return;
    if (a <= l && r <= b) {
      lazy[k] += v;
      return;
    }
    if (lazy[k]) {
      lazy[k * 2 + 1] += lazy[k];
      lazy[k * 2 + 2] += lazy[k];
      lazy[k] = 0;
    }
    add(a, b, v, k * 2 + 1, l, (l + r) / 2);
    add(a, b, v, k * 2 + 2, (l + r) / 2, r);
    dat[k] =
        min(dat[k * 2 + 1] + lazy[k * 2 + 1], dat[k * 2 + 2] + lazy[k * 2 + 2]);
  }
  T query(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return inf;
    if (a <= l && r <= b)
      return dat[k] + lazy[k];
    if (lazy[k]) {
      lazy[k * 2 + 1] += lazy[k];
      lazy[k * 2 + 2] += lazy[k];
      lazy[k] = 0;
    }
    T vl = query(a, b, k * 2 + 1, l, (l + r) / 2);
    T vr = query(a, b, k * 2 + 2, (l + r) / 2, r);
    return min(vl, vr);
  }
  T query(int a, int b) { return query(a, b, 0, 0, n); }
  void add(int a, int b, ll v) { add(a, b, v, 0, 0, n); }
};

int main() {
  cin.tie(0);
  cin.sync_with_stdio(0);

  int n, len;
  cin >> n >> len;
  string s, t;
  cin >> s >> t;

  vi ans(n + 1);
  SegTree<int> seg(n + 2);
  rep(i, n) {
    int best = inf;
#if 0
		if(s[i] == t[i]) best = ans[i];
		//for(int j = i; j >= 0 && s[j] == t[j]; j--) best = min(best, ans[j]);
		if(i == 0 || t[i - 1] != t[i]) seg.add(0, i + 1, 1);
		best = min(best, seg.query(max(0, i + 1 - len), i + 1) / 2 + 1);
		
		seg.add(i + 1, i + 2, 2 * best);
#endif

    if (s[i] == t[i])
      best = ans[i];
    int col = 1;
    for (int j = i; j >= 0 && j > i - len; j--) {
      if (j < i && t[j] != t[j + 1])
        col++;
      best = min(best, ans[j] + col / 2 + 1);
    }
    ans[i + 1] = best;
  }
  cout << ans[n] << endl;
  return 0;
}