leetcode-go/poj/1077.eight/main.cc

#include <cstdio>
#include <cstdlib>
#include <cstring>

const int dst = 123456780;
const int S = 362880;
const int frac[9] = {1, 1, 2, 6, 24, 120, 720, 5040, 40320};

int perm2no(int p) {
  char perm[10];
  sprintf(perm, "%09d", p);
  bool used[9] = {};
  int n = 0;
  for (int i = 0; i < 9; i++) {
    for (int j = perm[i] - '1'; 0 <= j; j--) {
      if (used[j]) continue;
      n += frac[8 - i];
    }
    used[perm[i] - '0'] = true;
  }
  return n;
}

int no2perm(int n) {
  char perm[10];
  bool used[9] = {};
  for (int i = 0; i < 9; i++) {
    int j = n / frac[8 - i];
    n %= frac[8 - i];
    for (int k = 0; k < 9; k++) {
      if (used[k]) continue;
      if (j-- == 0) {
        used[k] = true;
        perm[i] = k + '0';
        break;
      }
    }
  }
  return atoi(perm);
}

int next_state(int p, char m) {
  char perm[10];
  sprintf(perm, "%09d", p);
  int i = 0;
  while (perm[i] != '0') i++;
  switch (m) {
    case 'u':
      if (i < 3) return -1;
      perm[i] = perm[i - 3];
      perm[i - 3] = '0';
      return atoi(perm);
    case 'd':
      if (8 < i + 3) return -1;
      perm[i] = perm[i + 3];
      perm[i + 3] = '0';
      return atoi(perm);
    case 'l':
      if (i % 3 == 0) return -1;
      perm[i] = perm[i - 1];
      perm[i - 1] = '0';
      return atoi(perm);
    case 'r':
      if ((i + 1) % 3 == 0) return -1;
      perm[i] = perm[i + 1];
      perm[i + 1] = '0';
      return atoi(perm);
  }
  return -1;
}

struct Node {
  int sta;
  int pre;
  char mov;
  Node(int sta = 0, int pre = 0, char mov = 0) : sta(sta), pre(pre), mov(mov) {}
} queue[S];

int head, tail;

bool used[S];

char moves[] = "udlr";
char res[S + 1];

bool bfs(int s, int t) {
  head = 0, tail = 1;
  queue[head] = Node(s, -1);
  memset(used, 0, sizeof(used));
  used[perm2no(s)] = true;
  while (head != tail) {
    Node &cur = queue[head];
    if (cur.sta == t) return true;
    for (int i = 0; i < 4; i++) {
      char m = moves[i];
      int nxt = next_state(cur.sta, m);
      if (nxt == -1) continue;
      int no = perm2no(nxt);
      if (used[no]) continue;
      queue[tail++] = Node(nxt, head, m);
      used[no] = true;
    }
    head++;
  }
  return false;
}

int main() {
  char perm[10];
  for (int i = 0; i < 9; i++) {
    scanf("%s", &perm[i]);
    if (perm[i] == 'x') perm[i] = '0';
  }
  int src = atoi(perm);
  if (bfs(src, dst)) {
    int pos = head, i = 0;
    do {
      res[i++] = queue[pos].mov;
      pos = queue[pos].pre;
    } while (pos != -1);
    for (int j = i - 1; 0 <= j; j--) printf("%c", res[j]);
    printf("\n");
  } else {
    printf("unsolvable\n");
  }
  return 0;
}