Problem p00481 (C++) — buggy submission, verdict: Runtime Error

#include <iostream>
#include <queue>
using namespace std;
typedef pair<int, int> P;
const int MAX_HW = 1000;
int n, h, w, rat, s[2], g[2], flag[MAX_HW + 1][MAX_HW + 1][2];
int town[MAX_HW][MAX_HW];
char town2[MAX_HW][MAX_HW];
queue<P> que;
P p;
void bfs(int sy, int sx) {
  que.push(P(sy, sx));
  flag[sy][sx][0] = rat;
  while (que.size()) {
  RE:
    p = que.front();
    que.pop();
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        if (i == 0 || j == 0) {
          if (town2[p.first + i][p.second + j] != 'X' &&
              flag[p.first + i][p.second + j][0] != rat && p.first + i < h &&
              p.first + i >= 0 && p.second + j < w && p.second + j >= 0) {
            flag[p.first + i][p.second + j][0] = rat;
            flag[p.first + i][p.second + j][1] = flag[p.first][p.second][1] + 1;
            if (town[p.first + i][p.second + j] == rat) {
              if (rat == n)
                return;
              rat++;
              flag[p.first + i][p.second + j][0] = rat;
              while (que.size()) {
                que.pop();
              }
              que.push(P(p.first + i, p.second + j));
              goto RE;
            } else {
              que.push(P(p.first + i, p.second + j));
            }
          }
        }
      }
    }
  }
}
int main() {
  cin >> h >> w >> n;
  rat = 1;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      cin >> town2[i][j];
      town[i][j] = town2[i][j] - 48;
      if (town2[i][j] == 'S') {
        s[0] = i;
        s[1] = j;
      }
      if (town[i][j] == n) {
        g[0] = i;
        g[1] = j;
      }
    }
  }
  bfs(s[0], s[1]);
  cout << flag[g[0]][g[1]][1] << endl;
  return 0;
}

Error: 0

Fixed submission:

#include <iostream>
#include <queue>
using namespace std;
typedef pair<int, int> P;
const int MAX_HW = 1000;
int n, h, w, rat, s[2], g[2], flag[MAX_HW][MAX_HW][2];
int town[MAX_HW][MAX_HW];
char town2[MAX_HW][MAX_HW];
queue<P> que;
P p;
void bfs(int sy, int sx) {
  que.push(P(sy, sx));
  flag[sy][sx][0] = rat;
  while (que.size()) {
  RE:
    p = que.front();
    que.pop();
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        if (i == 0 || j == 0) {
          if (town2[p.first + i][p.second + j] != 'X' &&
              flag[p.first + i][p.second + j][0] != rat && p.first + i < h &&
              p.first + i >= 0 && p.second + j < w && p.second + j >= 0) {
            flag[p.first + i][p.second + j][0] = rat;
            flag[p.first + i][p.second + j][1] = flag[p.first][p.second][1] + 1;
            if (town[p.first + i][p.second + j] == rat) {
              if (rat == n)
                return;
              rat++;
              flag[p.first + i][p.second + j][0] = rat;
              while (que.size()) {
                que.pop();
              }
              que.push(P(p.first + i, p.second + j));
              goto RE;
            } else {
              que.push(P(p.first + i, p.second + j));
            }
          }
        }
      }
    }
  }
}
int main() {
  cin >> h >> w >> n;
  rat = 1;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      cin >> town2[i][j];
      town[i][j] = town2[i][j] - 48;
      if (town2[i][j] == 'S') {
        s[0] = i;
        s[1] = j;
      }
      if (town[i][j] == n) {
        g[0] = i;
        g[1] = j;
      }
    }
  }
  bfs(s[0], s[1]);
  cout << flag[g[0]][g[1]][1] << endl;
  return 0;
}