Problem p00941 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>

#define _overload(_1, _2, _3, name, ...) name
#define _rep(i, n) _range(i, 0, n)
#define _range(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rep(...) _overload(__VA_ARGS__, _range, _rep, )(__VA_ARGS__)

#define _rrep(i, n) _rrange(i, n, 0)
#define _rrange(i, a, b) for (int i = (int)(a)-1; i >= (int)(b); --i)
#define rrep(...) _overload(__VA_ARGS__, _rrange, _rrep, )(__VA_ARGS__)

#define _all(arg) begin(arg), end(arg)
#define uniq(arg) sort(_all(arg)), (arg).erase(unique(_all(arg)), end(arg))
#define getidx(ary, key) lower_bound(_all(ary), key) - begin(ary)
#define clr(a, b) memset((a), (b), sizeof(a))
#define bit(n) (1LL << (n))

// #define DEBUG

#ifdef DEBUG
#define dump(...) fprintf(stderr, __VA_ARGS__)
#else
#define dump(...)
#endif

template <class T> bool chmax(T &a, const T &b) {
  return (a < b) ? (a = b, 1) : 0;
}
template <class T> bool chmin(T &a, const T &b) {
  return (b < a) ? (a = b, 1) : 0;
}

using namespace std;
using ll = long long;
using vi = vector<int>;
using vll = vector<ll>;

const double EPS = 1e-10;
const double PI = acos(-1.0);
const ll inf = 1LL << 62;
const ll mod = 1000000007LL;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

ll extgcd(ll a, ll b, ll &x, ll &y) {
  x = 1, y = 0;
  ll g = a;
  if (b != 0)
    g = extgcd(b, a % b, y, x), y -= a / b * x;
  return g;
}
ll ADD(const ll &a, const ll &b, const ll &mod) { return (a + b) % mod; }
ll SUB(const ll &a, const ll &b, const ll &mod) { return (a - b + mod) % mod; }
ll MUL(const ll &a, const ll &b, const ll &mod) { return (1LL * a * b) % mod; }
ll DIV(const ll &a, const ll &b, const ll &mod) {
  ll x, y;
  extgcd(b, mod, x, y);
  return MUL(a, (x + mod) % mod, mod);
}

random_device rd;
mt19937 mt(rd());
uniform_int_distribution<int> dice(1, 6);
uniform_real_distribution<double> score(0.0, 10.0);

ll K;
string in, rev;
int n, optlen;
vector<vi> dp1;
vector<vll> dp2;

ll addOcc(ll a, ll b) { return min(K, a + b); }

ll rec(int r, int c) {
  ll &ret = dp2[r][c];
  if (ret != -1) {
    return ret;
  }

  int len = dp1[r][c];
  ret = 0;
  if (r < n and c < n and in[r] == rev[c]) {
    assert(dp1[r + 1][c + 1] == len + 1);
    ret = rec(r + 1, c + 1);
  } else {
    if (r < n and dp1[r + 1][c] == len + 1) {
      ret = addOcc(ret, rec(r + 1, c));
    }
    if (c < n and dp1[r][c + 1] == len + 1) {
      ret = addOcc(ret, rec(r, c + 1));
    }
  }

  if (len >= (optlen + 1) / 2) {
    chmin(ret, 1LL);
  }

  return ret;
}

string res;
int cnt = 0;
void explore(int r, int c, ll rest) {
  if (cnt == (optlen + 1) / 2)
    return;
  if (cnt != dp1[r][c]) {
    cerr << res << endl;
    cerr << in.substr(r - 1, 3) << ", " << rev.substr(c - 1, 3) << endl;
    assert(false);
  }
  cnt++;

  assert(rest >= 1LL);

  // stringstream ss; ss << (min(rest, 99LL)); res += ss.str();

  int len = dp1[r][c];

  // cerr << res << endl;
  if (in[r] == rev[c]) {
    assert(dp1[r + 1][c + 1] == len + 1);
    assert(dp2[r][c] == dp2[r + 1][c + 1]);
    res += in[r];
    explore(r + 1, c + 1, rest);
    return;
  }

  if (r < n and dp1[r + 1][c] != len + 1)
    dp2[r + 1][c] = 0;
  if (c < n and dp1[r][c + 1] != len + 1)
    dp2[r][c + 1] = 0;
  // assert(dp2[r][c] == addOcc(dp2[r + 1][c], dp2[r][c + 1]));
  /*
  if(rest == 1){
      if(r < n and dp2[r + 1][c] >= 1){
          res += in[r];
          explore(r + 1, c, rest);
      }
      else {
          res += rev[c];
          explore(r, c + 1, rest);
      }
      return;
  }
  */

  if (in[r] < rev[c]) {
    if (dp2[r + 1][c] >= rest) {
      res += in[r];
      explore(r + 1, c, rest);
    } else {
      res += rev[c];
      explore(r, c + 1, rest - dp2[r + 1][c]);
    }
  } else {
    if (dp2[r][c + 1] >= rest) {
      res += rev[c];
      explore(r, c + 1, rest);
    } else {
      res += in[r];
      explore(r + 1, c, rest - dp2[r][c + 1]);
    }
  }
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  cin >> in;
  cin >> K;
  n = (int)in.size();
  rev = in;
  reverse(_all(rev));

  dp1 = vector<vi>(n + 1, vi(n + 1));
  rep(i, n + 1) dp1[i][0] = dp1[0][i] = i;
  rep(i, 1, n + 1) {
    rep(j, 1, n + 1) {
      int &cur = dp1[i][j];
      if (in[i - 1] == rev[j - 1]) {
        cur = dp1[i - 1][j - 1] + 1;
      } else {
        cur = min(dp1[i - 1][j] + 1, dp1[i][j - 1] + 1);
      }
    }
  }
  optlen = dp1[n][n];

  /*
  cerr << "dp1: " << endl;
  rep(i, n + 1){
      rep(j, n + 1){
          cerr << dp1[i][j] << " ";
      }
      cerr << endl;
  }
  cerr << endl;
  */

  dp2 = vector<vll>(n + 1, vll(n + 1, -1));
  dp2[n][n] = 1;
  rec(0, 0);
  rep(i, n + 1) rep(j, n + 1) if (dp2[i][j] == -1) dp2[i][j] = 0;

  /*
  cerr << "dp2: " << endl;
  rep(i, n + 1){
      rep(j, n + 1){
          cerr << dp2[i][j] << " ";
      }
      cerr << endl;
  }
  cerr << endl;
  //*/

  if (dp2[0][0] < K) {
    cout << "NONE" << endl;
    return 0;
  }
  explore(0, 0, K);

  string rres = res;
  reverse(_all(rres));

  if (optlen % 2 == 1) {
    cout << res + rres.substr(1) << endl;
  } else {
    cout << res + rres << endl;
  }

  return 0;
}

Error: 0
dp2: 
1 0 0 0 
0 1 0 0 
0 0 1 0 
0 0 0 1 



Fixed submission:

#include <bits/stdc++.h>

#define _overload(_1, _2, _3, name, ...) name
#define _rep(i, n) _range(i, 0, n)
#define _range(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rep(...) _overload(__VA_ARGS__, _range, _rep, )(__VA_ARGS__)

#define _rrep(i, n) _rrange(i, n, 0)
#define _rrange(i, a, b) for (int i = (int)(a)-1; i >= (int)(b); --i)
#define rrep(...) _overload(__VA_ARGS__, _rrange, _rrep, )(__VA_ARGS__)

#define _all(arg) begin(arg), end(arg)
#define uniq(arg) sort(_all(arg)), (arg).erase(unique(_all(arg)), end(arg))
#define getidx(ary, key) lower_bound(_all(ary), key) - begin(ary)
#define clr(a, b) memset((a), (b), sizeof(a))
#define bit(n) (1LL << (n))

// #define DEBUG

#ifdef DEBUG
#define dump(...) fprintf(stderr, __VA_ARGS__)
#else
#define dump(...)
#endif

template <class T> bool chmax(T &a, const T &b) {
  return (a < b) ? (a = b, 1) : 0;
}
template <class T> bool chmin(T &a, const T &b) {
  return (b < a) ? (a = b, 1) : 0;
}

using namespace std;
using ll = long long;
using vi = vector<int>;
using vll = vector<ll>;

const double EPS = 1e-10;
const double PI = acos(-1.0);
const ll inf = 1LL << 62;
const ll mod = 1000000007LL;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

ll extgcd(ll a, ll b, ll &x, ll &y) {
  x = 1, y = 0;
  ll g = a;
  if (b != 0)
    g = extgcd(b, a % b, y, x), y -= a / b * x;
  return g;
}
ll ADD(const ll &a, const ll &b, const ll &mod) { return (a + b) % mod; }
ll SUB(const ll &a, const ll &b, const ll &mod) { return (a - b + mod) % mod; }
ll MUL(const ll &a, const ll &b, const ll &mod) { return (1LL * a * b) % mod; }
ll DIV(const ll &a, const ll &b, const ll &mod) {
  ll x, y;
  extgcd(b, mod, x, y);
  return MUL(a, (x + mod) % mod, mod);
}

random_device rd;
mt19937 mt(rd());
uniform_int_distribution<int> dice(1, 6);
uniform_real_distribution<double> score(0.0, 10.0);

ll K;
string in, rev;
int n, optlen;
vector<vi> dp1;
vector<vll> dp2;

ll addOcc(ll a, ll b) { return min(K, a + b); }

ll rec(int r, int c) {
  ll &ret = dp2[r][c];
  if (ret != -1) {
    return ret;
  }

  int len = dp1[r][c];
  ret = 0;
  if (r < n and c < n and in[r] == rev[c]) {
    ret = rec(r + 1, c + 1);
  } else {
    if (r < n and dp1[r + 1][c] == len + 1) {
      ret = addOcc(ret, rec(r + 1, c));
    }
    if (c < n and dp1[r][c + 1] == len + 1) {
      ret = addOcc(ret, rec(r, c + 1));
    }
  }

  if (len >= (optlen + 1) / 2) {
    chmin(ret, 1LL);
  }

  return ret;
}

string res;
void explore(int r, int c, ll rest) {
  if ((int)res.size() == (optlen + 1) / 2)
    return;

  // cerr << res << endl;
  if (in[r] == rev[c]) {
    res += in[r];
    explore(r + 1, c + 1, rest);
    return;
  }

  /*
  if(rest == 1){
      if(r < n){
          res += in[r];
          explore(r + 1, c, rest);
      }
      else {
          res += rev[c];
          explore(r, c + 1, rest);
      }
      return;
  }
  */

  if (in[r] < rev[c]) {
    if (dp2[r + 1][c] >= rest) {
      res += in[r];
      explore(r + 1, c, rest);
    } else {
      res += rev[c];
      explore(r, c + 1, rest - dp2[r + 1][c]);
    }
  } else {
    if (dp2[r][c + 1] >= rest) {
      res += rev[c];
      explore(r, c + 1, rest);
    } else {
      res += in[r];
      explore(r + 1, c, rest - dp2[r][c + 1]);
    }
  }
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);

  cin >> in;
  cin >> K;
  n = (int)in.size();
  rev = in;
  reverse(_all(rev));

  dp1 = vector<vi>(n + 1, vi(n + 1));
  rep(i, n + 1) dp1[i][0] = dp1[0][i] = i;
  rep(i, 1, n + 1) {
    rep(j, 1, n + 1) {
      int &cur = dp1[i][j];
      if (in[i - 1] == rev[j - 1]) {
        cur = dp1[i - 1][j - 1] + 1;
      } else {
        cur = min(dp1[i - 1][j] + 1, dp1[i][j - 1] + 1);
      }
    }
  }
  optlen = dp1[n][n];

  /*
  cerr << "dp1: " << endl;
  rep(i, n + 1){
      rep(j, n + 1){
          cerr << dp1[i][j] << " ";
      }
      cerr << endl;
  }
  cerr << endl;
  */

  dp2 = vector<vll>(n + 1, vll(n + 1, -1));
  dp2[n][n] = 1;
  rec(0, 0);
  rep(i, n + 1) rep(j, n + 1) if (dp2[i][j] == -1) dp2[i][j] = 0;

  cerr << "dp2: " << endl;
  rep(i, n + 1) {
    rep(j, n + 1) { cerr << dp2[i][j] << " "; }
    cerr << endl;
  }
  cerr << endl;

  if (dp2[0][0] < K) {
    cout << "NONE" << endl;
    return 0;
  }
  explore(0, 0, K);

  string rres = res;
  reverse(_all(rres));

  if (optlen % 2 == 1) {
    cout << res + rres.substr(1) << endl;
  } else {
    cout << res + rres << endl;
  }

  return 0;
}