USACO 4.4 Shuttle Puzzle(dfs)

比较有意思的一个题，虽然不知道为什么这么做。。。看样例就可以发现一般是w_  _b  wb_ _bw只能这样4种操作。。。没想到就这样过了。。注意了一下20个换行，2Y。。。我开始还在想bfs还想过，直接构造，构造不出来。。。

  1 /*

  2 ID:cuizhe

  3 LANG: C++

  4 TASK: shuttle

  5 */

  6 #include <iostream>

  7 #include <cstdio>

  8 #include <map>

  9 #include <algorithm>

 10 #include <cstring>

 11 #include <string>

 12 using namespace std;

 13 map<string,int> mp;

 14 char str[32];

 15 int que[100001];

 16 int a[100001];

 17 int ans = 10000000;

 18 int n;

 19 int judge()

 20 {

 21     int i;

 22     for(i = 0; i < n; i ++)

 23         if(str[i] != '2')

 24             break;

 25     if(i == n&&str[n] == ' ')

 26         return 1;

 27     else

 28         return 0;

 29 }

 30 void dfs(int x,int step)

 31 {

 32     int i;

 33     if(ans <= step) return;

 34     if(judge())

 35     {

 36         if(step-1 < ans)

 37         {

 38             ans = step-1;

 39             for(i = 1;i <= ans;i ++)

 40             a[i] = que[i];

 41         }

 42         return ;

 43     }

 44     if(x-1 >= 0&&x-2 >= 0&&str[x-1] == '2'&&str[x-2] == '1')

 45     {

 46         que[step] = x-2;

 47         str[x-2] = ' ';

 48         str[x] = '1';

 49         dfs(x-2,step+1);

 50         str[x-2] = '1';

 51         str[x] = ' ';

 52     }

 53     if(x-1 >= 0&&str[x-1] == '1')

 54     {

 55         que[step] = x-1;

 56         str[x] = '1';

 57         str[x-1] = ' ';

 58         dfs(x-1,step+1);

 59         str[x] = ' ';

 60         str[x-1] = '1';

 61     }

 62     if(x+1 <= 2*n&&x+2 <= 2*n&&str[x+1] == '1'&&str[x+2] == '2')

 63     {

 64         que[step] = x+2;

 65         str[x+2] = ' ';

 66         str[x] = '2';

 67         dfs(x+2,step+1);

 68         str[x+2] = '2';

 69         str[x] = ' ';

 70     }

 71     if(x+1 <= 2*n&&str[x+1] == '2')

 72     {

 73         que[step] = x+1;

 74         str[x] = '2';

 75         str[x+1] = ' ';

 76         dfs(x+1,step+1);

 77         str[x] = ' ';

 78         str[x+1] = '2';

 79     }

 80     return ;

 81 }

 82 int main()

 83 {

 84     int i;

 85     freopen("shuttle.in","r",stdin);

 86     freopen("shuttle.out","w",stdout);

 87     scanf("%d",&n);

 88     for(i = 0; i < n; i ++)

 89     {

 90         str[i] = '1';

 91         str[n+i+1] = '2';

 92     }

 93     str[n] = ' ';

 94     dfs(n,1);

 95     for(i = 1;i <= ans;i ++)

 96     {

 97         if(i%20 == 0||i == ans)

 98         printf("%d\n",a[i]+1);

 99         else

100         printf("%d ",a[i]+1);

101     }

102     return 0;

103 }