【HDOJ】3500 Fling

题意巨难懂。简言之，就是球互相碰撞时，主动碰撞的球将会停止，另一个球将沿着碰撞方向继续移动，不断碰撞。但是无法弹射紧挨着的球，但是若a弹射b，bc相邻，这种情况b可以弹射c。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;

#define MAXN 10

typedef struct {
    int x, y;
    char d;
} node_t;

node_t nodes[15];
char map[MAXN][MAXN];
int on;
// U < L < R < D.
int dir[4][2] = {-1,0,0,-1,0,1,1,0};

bool check(int x, int y) {
    return x<0||x>=7||y<0||y>=8;
}

char getDir(int k) {
    if (k==0) return 'U';
    if (k==1) return 'L';
    if (k==2) return 'R';
    return 'D';
}

bool dfs(int n) {
    int i, j, k, x, y;
    bool flag;

    if (n == 0)
        return true;

    for (i=0; i<7; ++i) {
        for (j=0; j<8; ++j) {
            if (map[i][j] == 'O') {
                for (k=0; k<4; ++k) {
                    x = i + dir[k][0];
                    y = j + dir[k][1];
                    if (check(x, y) || map[x][y]=='O')
                        continue;
                    flag = false;
                    while (!check(x, y)) {
                        if (map[x][y] == 'O') {
                            flag = true;
                            map[x][y] = 'X';
                            map[x-dir[k][0]][y-dir[k][1]] = 'O';
                        }
                        x += dir[k][0];
                        y += dir[k][1];
                    }
                    if (flag) {
                        map[i][j] = 'X';
                        nodes[n].x = i;
                        nodes[n].y = j;
                        nodes[n].d = getDir(k);
                        if (dfs(n-1))
                            return true;
                        do {
                            x -= dir[k][0];
                            y -= dir[k][1];
                            if (map[x][y] == 'O') {
                                map[x][y] = 'X';
                                map[x+dir[k][0]][y+dir[k][1]] = 'O';
                            }
                        } while (x!=i || y!=j);
                        map[i][j] = 'O';
                    }
                }
            }
        }
    }

    return false;
}

int main() {
    int i = 0, j;
    int t = 0;

#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif

    while (scanf("%s", &map[i]) != EOF) {
        for (i=1; i<7; ++i)
            scanf("%s", &map[i]);
        on = 0;
        for (i=0; i<7; ++i)
            for (j=0; j<8; ++j)
                if (map[i][j] == 'O')
                    ++on;
        dfs(on-1);

        if (t != 0)
            printf("
");
        printf("CASE #%d:
", ++t);
        for(i=on-1; i>=1; --i)
            printf("%d %d %c
", nodes[i].x, nodes[i].y, nodes[i].d);
        getchar();
        i = 0;
    }

    return 0;
}