use the symmetry of the problem and reduce it. let F, T denotes the two sides of the coin.
We use the following system: (x,y) means x row, yth coin counting from the left.
WLOG, consider T(3,2), F(4,2), F(4,3), so T(5,3). Now we must have T(4,1) and T(4,4). So we have F(1,1). One of (3,1), (3,3) is F. Let's have F(3,1), so T(3,3). then F(2,2), F(5,5), T(5,1), and F(5,2). Now, what to put on (2,2)?
Start with F(3,2), T(4,2), T(4,3), then F(4,1), F(4,4). So we have T(1,1). One of (3,1), (3,3) is F. Let's have F(3,1), then T(3,3). So T(2,1), F(2,2).
Now only the last row is undetermined.
If T(5,1), then F(5,5). How to put (5,4)?
If F(5,1), then T(5,2), How to put (5,3)?www.ddhw.com
哈哈哈哈 O(∩_∩)O哈哈~ 好久不来了，不小心看了一眼自己主页，发现自动关联了自己以往发过的帖，点进这个看了一眼，笑个半死。。。敢情我当初还发过这么搞笑的东西啊，我自己完全不记得了 
雷达卡
发表于 2005-4-12 05:46:35