POJ 1324(BFS + 状态压缩)

题意：给你一条蛇，要求一以最少的步数走到1,1

思路：

最开始一直没想到应该怎样保存状态，后来发现别人用二进制保存蛇的状态，即每两个节点之间的方向和头节点，二进制最多14位（感觉状态保存都能扯到二进制）。然后就是bfs

问题：

1.最开始完全没想到状态压缩的问题

2.感觉现在做题太急，做题没有足够的思考，思路不清晰便开始写，导致在过程中经常崩盘。

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <queue>
#include <algorithm>
typedef long long ll;
using namespace std;
int flag;
const int maxn = 25;
const int ma = (1<<14)+10;
int vis[maxn][maxn];
int sta[maxn][maxn][ma];
struct node
{
//    int snake[15][2];
    int x,y;
    int step;
    int state;
};
node cur;
int t,n,m;
int dir[4][2] = {{-1,0},{0,-1},{1,0},{0,1}};
int matri[20][2];
int get_state()   //蛇的状态
{
    int dir;
    int stat = 0;
    for(int i = t; i > 1; i --)
    {
        int x = matri[i][0]-matri[i-1][0];
        int y = matri[i][1]-matri[i-1][1];
        if(x == -1 && y == 0)
            dir = 0;
        if(x == 0 && y == -1)
            dir = 1;
        if(x == 1 && y == 0)
            dir =2 ;
        if(x == 0 && y == 1)
            dir = 3;
        stat <<= 2;
        stat = stat |dir;
    }
    return stat;
}

bool judge(int x,int y,int x1,int y1,int  state)        //是否吃自己
{
    int s;
    for(int i = 1; i < t; i++)
    {
        s = 3;
        s = state & s;            //取最后两位
        state = state >> 2;
        if(x == x1+dir[s][0] && y == y1+dir[s][1])
            return true;
        x1  = x1 + dir[s][0];
        y1  = y1 + dir[s][1];
    }
    return false ;
}

int get_next(int i,int state)       //去掉高位，输入低位
{
    int x = 0-dir[i][0];
    int y = 0-dir[i][1];
    int dir;

    int k = (1 << ((t - 1) << 1)) - 1;
    if(x == -1 && y == 0)
        dir = 0;
    if(x == 0 && y == -1)
        dir = 1;
    if(x == 1 && y == 0)
        dir =2 ;
    if(x == 0 && y == 1)
        dir = 3;
    state <<= 2;
    state |= dir;
    state = state & k;        //去掉最高位
    return state;
}

void bfs()
{
    queue<node>q;
    cur.step = 0;
    sta[cur.x][cur.y][cur.state] = 1;
    q.push(cur);
    node fo;
    while(!q.empty())
    {
        fo = q.front();
        q.pop();

        for(int i = 0; i < 4; i++)
        {
            int x = fo.x+dir[i][0];
            int y = fo.y+dir[i][1];
            int ts = get_next(i,fo.state);
            if(x < 1 || x > n || y <1 || y > m || vis[x][y] || judge(x,y,fo.x,fo.y,fo.state) || sta[x][y][ts])
                continue;
            if(x == 1 && y == 1)
            {
                printf("%d\n",fo.step+1);
                flag = 1;
                return;
            }
            node tmp;
            tmp.x =x ;
            tmp.y = y;
            tmp.step = fo.step + 1;
//            for(int i = t; i >= 1; i--)
//            {
//                tmp.snake[i][0] = tt.snake[i-1][0];
//                tmp.snake[i][1] = tt.snake[i-1][1];
//            }
            tmp.state = ts;
            sta[x][y][tmp.state] = 1;
            q.push(tmp);
        }
    }
}

int main()
{
    int cas= 1;
    while(scanf("%d%d%d",&n,&m,&t) != EOF)
    {
        memset(vis,0,sizeof(vis));
        memset(sta,0,sizeof(sta));
        if(n == 0 && m == 0 && t == 0)
            break;
        for(int i = 1; i <= t; i++)
            scanf("%d%d",&matri[i][0],&matri[i][1]);

        int a,b,k;
        scanf("%d",&k);
        for(int i = 0; i < k; i++)
        {
            scanf("%d%d",&a,&b);
            vis[a][b] = 1;
        }
        cur.x = matri[1][0];
        cur.y = matri[1][1];
        cur.state = get_state();
        flag = 0;
        printf("Case %d: ",cas++);
        if(cur.x == 1 && cur.y == 1)
        {
            printf("0\n");
            continue;
        }
        bfs();
        if(!flag)
            printf("-1\n");
    }
}