【luogu P2324 [SCOI2005]骑士精神】 题解

题目链接：https://www.luogu.org/problemnew/show/P2324

不懂怎么剪枝，所以说，，我需要氧气。。

第一道A*

// luogu-judger-enable-o2
#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#define maxn 10
using namespace std;
int T, ans = 16, mx, sx, sy;
int aim[6][6] = {{0,0,0,0,0,0},
                 {0,2,2,2,2,2},
                 {0,1,2,2,2,2},
                 {0,1,1,0,2,2},
                 {0,1,1,1,1,2},
                 {0,1,1,1,1,1}};
int dx[8] = {-2,-1,1,2,2,1,-1,-2};
int dy[8] = {1,2,2,1,-1,-2,-2,-1};
int a[maxn][maxn];
inline int h()
{
    int ret = 0;
    for(int i = 1; i <= 5; i++)
    for(int j = 1; j <= 5; j++)
    if(a[i][j] != aim[i][j]) ret++;
    return ret;
}
inline bool move(int x, int y)
{
    if(x >= 1 && x <= 5 && y >= 1 && y <= 5) return true;
    return false; 
}
void A_star(int now, int x, int y)
{
    if(now > mx) return;
    int cnt = h();
    if(cnt == 0)
    {
        ans = min(ans, now);
        return;
    }
    if(cnt + now - 1 > mx) return;
    int nowx, nowy;
    for(int i = 0; i <= 7; i++)
    {
        nowx = x + dx[i];
        nowy = y + dy[i];
        if(move(nowx, nowy))
        {
            swap(a[nowx][nowy],a[x][y]);
            A_star(now+1,nowx,nowy);
            swap(a[nowx][nowy],a[x][y]);
        }
    }
}
int main()
{
    cin.sync_with_stdio(false);
    cin>>T;
    while(T--)
    {
        char s;
        ans = 16;
        memset(a,0,sizeof(a));
        for(int i = 1; i <= 5; i++)
        for(int j = 1; j <= 5; j++)
        {
            cin>>s;
            if(s != '*')
            a[i][j] = s + 1 - '0';
            if(s == '*')
            {
                a[i][j] = 0;
                sx = i;
                sy = j;
            }
        }
        for(mx = 1; mx <= 15; mx++)
        {
            A_star(0,sx,sy);
            if(ans == mx) break;
            
        }
        if(ans == 16)
        {
            cout<<-1<<endl;
        }
        else 
        {
            cout<<ans<<endl;
        }
    }
    return 0;
}