UVa 10085 - The most distant state

八数码问题的变形，bfs+哈希，打印路径时花了点时间，用哈希表next数组的原理进行回退就可以将问题解决。

代码如下：

#include <iostream>#include <algorithm>#include <cstring>#include <cstdlib>#include <cstdio>#include <cmath>using namespace std;const int MAXSIZE = 1000003;const int dx[4] = {-1, 1, 0, 0};const int dy[4] = {0, 0, -1, 1};int back[MAXSIZE], path[MAXSIZE][9], dist[MAXSIZE];int head[MAXSIZE], next[MAXSIZE];char ac[MAXSIZE], save[100];int Hash(int s){    int v = 0;    for(int i=0; i<9; i++)        v = v * 10 + path[s][i];    return v % MAXSIZE;}int insert(int s){    int h = Hash(s);    int u = head[h];    while(u)    {        if(!memcmp(path[u], path[s], sizeof(path[s])))            return 0;        u = next[u];    }    next[s] = head[h];    head[h] = s;    return 1;}int bfs(){    memset(head, 0, sizeof(head));    int front = 1, rear = 2;    while(front < rear)    {        int z;        for(z=0; z<9; z++)            if(!path[front][z])                break;        int x = z/3, y = z%3;        for(int i=0; i<4; i++)        {            int newx = x + dx[i], newy = y +dy[i];            int newz = newx * 3 +newy;            if(newx >= 0 && newx < 3 && newy >= 0 && newy < 3)            {                memcpy(path[rear], path[front], sizeof(path[front]));                path[rear][newz] = path[front][z];                path[rear][z] = path[front][newz];                dist[rear] = dist[front] + 1;                if(insert(rear))                {                    back[rear] = front;                    switch(i)                    {                    case 0:                        ac[rear] = 'U';                        break;                    case 1:                        ac[rear] = 'D';                        break;                    case 2:                        ac[rear] = 'L';                        break;                    case 3:                        ac[rear] = 'R';                        break;                    }                    rear++;                }            }        }        ++front;    }    return rear - 1;}int main(){#ifdef test    freopen("sample.txt", "r", stdin);#endif    int num, goal;    scanf("%d", &num);    for(int i=1; i<=num; i++)    {        int sum = 0;        back[1] = 0;        for(int j=0; j<9; j++)            scanf("%d", &path[1][j]);        goal = bfs();        printf("Puzzle #%d\n", i);        for(int j=0; j<3; j++)        {            printf("%d", path[goal][j*3]);            for(int k=1; k<3; k++)                printf(" %d", path[goal][j*3+k]);            puts("");        }        while(goal)        {            save[++sum] = ac[goal];            goal = back[goal];        }        for (int i=sum-1; i; i--)            printf("%c", save[i]);        printf("\n\n");    }    return 0;}