python/TangDou/AcWing/BFS/AStar/179_TwoWaySearch_WithoutPath_845.cpp

#include <bits/stdc++.h>
using namespace std;
typedef unordered_map<string, int> MSI;
string st, ed = "12345678x";

int dx[] = {-1, 0, 1, 0};
int dy[] = {0, 1, 0, -1};

// AC掉AcWing 845, 运行时间：1440 ms
int extend(queue<string> &qa, MSI &da, MSI &db) {
    string t = qa.front();
    qa.pop();

    int k = t.find('x');
    int tx = k / 3, ty = k % 3;

    for (int i = 0; i < 4; i++) {
        int x = tx + dx[i], y = ty + dy[i];
        if (x < 0 || x > 2 || y < 0 || y > 2) continue;
        string v = t;
        swap(v[k], v[x * 3 + y]);
        if (da[v]) continue;
        if (db[v]) return da[t] + 1 + db[v];
        da[v] = da[t] + 1;
        qa.push(v);
    }
    return -1;
}

int bfs() {
    //不加特判挂一个点　
    if (st == ed) return 0;

    //双队列
    queue<string> qa, qb;
    //双距离
    MSI da, db;

    //将起点放入队列
    qa.push(st);
    da[st] = 0;

    //将终点放入队列
    qb.push(ed);
    db[ed] = 0;

    int t;
    //两个队列均不空
    while (qa.size() && qb.size()) {
        //双队列优化策略：哪个小就优先拓展哪个
        if (qa.size() <= qb.size())
            t = extend(qa, da, db);
        else
            t = extend(qb, db, da);
        if (t > 0) break;
    }
    return t; //返回最短距离
}

int main() {
    //加快读入
    cin.tie(0), ios::sync_with_stdio(false);

    char c;
    for (int i = 0; i < 9; i++) cin >> c, st += c;

    //八数码定理
    int nums = 0;
    for (int i = 0; i < 9; i++) {
        if (st[i] == 'x') continue;
        for (int j = i + 1; j < 9; j++) {
            if (st[j] == 'x') continue;
            if (st[j] < st[i]) nums++;
        }
    }

    //奇数个逆序对输出-1
    if (nums & 1)
        puts("-1");
    else
        //输出最短距离
        printf("%d\n", bfs());

    return 0;
}