Problem p02246 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;

static const int N = 4;
static const int N2 = 16;
static const int LIMIT = 100;

static const int dx[4] = {0, -1, 0, 1};
static const int dy[4] = {1, 0, -1, 0};
static const char dir[4] = {'r', 'u', 'l', 'd'};
int MDT[N2][N2];

struct Puzzle {
  int f[N2], space, MD;
};

Puzzle state;
int limit;
int path[LIMIT];

int getAllMD(Puzzle pz) {
  int sum = 0;
  for (int i = 0; i < N2; i++) {
    if (pz.f[i] == N2)
      continue;
    sum += MDT[i][pz.f[i] - 1];
  }
  return sum;
}

bool isSolved() {
  for (int i = 0; i < N2; i++) {
    if (state.f[i] != i + 1)
      return false;
  }
  return true;
}

bool dfs(int depth, int prev) {
  if (state.MD == 0)
    return true;
  if (depth + state.MD > limit)
    return false;

  int sx = state.space / N;
  int sy = state.space % N;
  Puzzle tmp;

  for (int r = 0; r < 4; r++) {
    int tx = sx + dx[r];
    int ty = sy + dy[r];
    if (tx < 0 || ty < 0 || tx >= N || ty >= N)
      continue;
    if ((max(prev, r) - min(prev, r)) == 2)
      continue;

    tmp = state;
    state.MD -= MDT[tx * N + ty][state.f[tx * N + ty] - 1];
    state.MD += MDT[sx * N + sy][state.f[tx * N + ty] - 1];
    swap(state.f[tx * N + ty], state.f[sx * N + sy]);
    state.space = tx * N + ty;
    if (dfs(depth + 1, r)) {
      path[depth] = r;
      return true;
    }
    state = tmp;
  }
  return false;
}

string iterative_deepening(Puzzle in) {
  in.MD = getAllMD(in);

  for (limit = in.MD; limit <= LIMIT; limit++) {
    state = in;
    if (dfs(0, -100)) {
      string ans = "";
      for (int i = 0; i < limit; i++)
        ans += dir[path[i]];
      return ans;
    }
  }
  return "Unsolvable";
}

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

  for (int i = 0; i < N2; i++) {
    for (int j = 0; j < N2; j++) {
      MDT[i][j] = abs(i / N - j / N) + abs(i % N - j % N);
    }
  }

  Puzzle in;
  for (int i = 0; i < N2; i++) {
    cin >> in.f[i];
    if (in.f[i] == 0) {
      in.f[i] = N2;
      in.space = i;
    }
  }

  string ans = iterative_deepening(in);
  cout << ans.size() << endl;

  return 0;
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;

static const int N = 4;
static const int N2 = 16;
static const int LIMIT = 100;

static const int dx[4] = {0, -1, 0, 1};
static const int dy[4] = {1, 0, -1, 0};
static const char dir[4] = {'r', 'u', 'l', 'd'};
int MDT[N2][N2];

struct Puzzle {
  int f[N2], space, MD;
  int cost;

  bool operator<(const Puzzle &p) const {
    for (int i = 0; i < N2; i++) {
      if (f[i] == p.f[i])
        continue;
      return f[i] < p.f[i];
    }
    return false;
  }
};

struct State {
  Puzzle puzzle;
  int estimated;
  bool operator<(const State &s) const { return estimated > s.estimated; }
};

int getAllMD(Puzzle pz) {
  int sum = 0;
  for (int i = 0; i < N2; i++) {
    if (pz.f[i] == N2)
      continue;
    sum += MDT[i][pz.f[i] - 1];
  }
  return sum;
}

int astar(Puzzle s) {
  priority_queue<State> PQ;
  s.MD = getAllMD(s);
  s.cost = 0;
  map<Puzzle, bool> V;
  Puzzle u, v;
  State initial;
  initial.puzzle = s;
  initial.estimated = getAllMD(s);
  PQ.push(initial);

  while (!PQ.empty()) {
    State st = PQ.top();
    PQ.pop();
    u = st.puzzle;

    if (u.MD == 0)
      return u.cost;
    V[u] = true;

    int sx = u.space / N;
    int sy = u.space % N;

    for (int r = 0; r < 4; r++) {
      int tx = sx + dx[r];
      int ty = sy + dy[r];
      if (tx < 0 || ty < 0 || sx >= N || sy >= N)
        continue;
      v = u;

      v.MD -= MDT[tx * N + ty][v.f[tx * N + ty] - 1];
      v.MD += MDT[sx * N + sy][v.f[tx * N + ty] - 1];
      swap(v.f[sx * N + sy], v.f[tx * N + ty]);
      v.space = tx * N + ty;

      if (!V[v]) {
        v.cost++;
        State news;
        news.puzzle = v;
        news.estimated = v.cost + v.MD;
        PQ.push(news);
      }
    }
  }
  return -1;
}

int main(void) {
  for (int i = 0; i < N2; i++) {
    for (int j = 0; j < N2; j++) {
      MDT[i][j] = abs(i / N - j / N) + abs(i % N - j % N);
    }
  }

  Puzzle in;
  for (int i = 0; i < N2; i++) {
    cin >> in.f[i];
    if (in.f[i] == 0) {
      in.f[i] = N2;
      in.space = i;
    }
  }

  cout << astar(in) << endl;

  return 0;
}