题目描述

强行以字典序最小顺序存储路径

算法1

C++ 代码

#include <iostream>
#include <algorithm>
#include <cstring>
#include <queue>
#include <unordered_map>

using namespace std;

unordered_map<string, char> ans;
unordered_map<string, string> pre;

int bfs(string start, string end)
{
    queue<string> q;
    unordered_map<string, int> d;

    q.push(start);
    d[start] = 0;

    int dx[4] = {1, 0, 0, -1}, dy[4] = {0, -1, 1, 0}; 
    char cs[] = "dlru";  // 下左右上

    while(q.size())
    {
        auto t = q.front();
        q.pop();

        int distance = d[t];

        if(t == end) return distance;

        int k = t.find('x');
        int x = k / 3, y = k % 3;
        pre[start] = "";
        for(int i = 0; i < 4; i ++ )
        {
            int a = x + dx[i], b = y + dy[i];
            if(a >= 0 && a < 3 && b >= 0 && b < 3)
            {
                swap(t[k], t[a * 3 + b]);

                if(d[t] == 0)
                {
                    d[t] = distance + 1;
                    q.push(t);

                    swap(t[k], t[a * 3 + b]);
                    string aa = t;
                    swap(t[k], t[a * 3 + b]);
                    string bb = t;
                    pre[bb] = aa;
                    ans[bb] = cs[i];

                }
                swap(t[k], t[a * 3 + b]);
            }
        }
    }
    return -1;
}

int main ()
{
    string start;
    for(int i = 0; i < 9; i ++ )
    {
        char c;
        cin >> c;
        start += c;
    }
    string end = "12345678x";
    int step = bfs(start, end);

    if(step == -1) cout << "unsolvable" << endl;
    else
    {
        string cnt = "";

        while(end != start)
        {
            cnt += ans[end];
            end = pre[end];
        }

        reverse(cnt.begin(), cnt.end());
        cout << cnt << endl;
    }

    return 0;
}