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

#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>

using namespace std;

#define N 4
#define N2 16
#define LIMIT 45

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

int MDT[N2][N2];

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

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

Puzzle state;
int limit;
int path[LIMIT];
bool dfs(int depth, int prevR) {
  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) {
    if (max(prevR, r) - min(r, prevR) == 2)
      continue;
    int tx = sx + dx[r];
    int ty = sy + dy[r];
    if (tx < 0 || ty < 0 || tx >= N || ty >= N)
      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[sx * N + sy], state.f[tx * N + ty]);
    state.space = tx * N + ty;
    if (dfs(depth + 1, r)) {
      path[depth] = r;
      return true;
    }
    state = tmp;
  }
  return false;
}

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

int main() {
  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 p;
  for (int i = 0; i < N2; ++i) {
    scanf("%d", &p.f[i]);
    if (p.f[i] == 0) {
      p.f[i] = N2;
      p.space = i;
    }
  }
  string ans = deepening(p);
  cout << ans.size() << endl;

  return 0;
}

Error: -11

Fixed submission:

// Iterative Deepening
#include "bits/stdc++.h"
using namespace std;
#define N 4
#define N2 16
#define LIMIT 45

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() {
  for (int i = 0; i < N2; ++i)
    for (int j = 0; j < N2; ++i)
      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;
}