八数码难题

Posted 2020-12-31 autoint

    // Rujia Liu
    #include<cstdio>
    #include<cstring>
    #include<set>
    using namespace std;
    typedef int State[9];
    const int MAXSTATE = 1000000;
    State st[MAXSTATE], goal={1,2,3,8,0,4,7,6,5};
    int dist[MAXSTATE]={0};
    set<int> vis;
    void init_lookup_table() { vis.clear(); }
    int try_to_insert(int s) {
      int v = 0;
      for(int i = 0; i < 9; i++) v = v * 10 + st[s][i];
      if(vis.count(v)) return 0;
      vis.insert(v);
      return 1;
    }
    const int dx[] = {-1, 1, 0, 0};
    const int dy[] = {0, 0, -1, 1};
    int bfs() {
      init_lookup_table();
      int front = 1, rear = 2;
      while(front < rear) {
        State& s = st[front];
        if(memcmp(goal, s, sizeof(s)) == 0) return front;
        int z;
        for(z = 0; z < 9; z++) if(!s[z]) break;
        int x = z/3, y = z%3;
        for(int d = 0; d < 4; d++) {
          int newx = x + dx[d];
          int newy = y + dy[d];
          int newz = newx * 3 + newy;
          if(newx >= 0 && newx < 3 && newy >= 0 && newy < 3) {
            State& t = st[rear];
            memcpy(&t, &s, sizeof(s));
            t[newz] = s[z];
            t[z] = s[newz];
            dist[rear] = dist[front] + 1;
            if(try_to_insert(rear)) rear++;
          }
        }
        front++;
      }
      return 0;
    }
    int main() {
      for(int i = 0; i < 9; i++)
        st[1][i]=getchar()-‘0‘;
      int ans = bfs();
      if(ans > 0) printf("%d
", dist[ans]);
      else printf("-1
");
      return 0;
}