uva639 回溯！

#include<iostream>

using namespace std;

int n,Max,C[4][4];
char board[5][5];
bool vis[16];
bool isok(int x,int y)
{
	for (int i = x + 1; i < n && board[i][y] != 'X'; i++)  
        if(board[i][y] == '0')  
            return false;  
    for (int i = x - 1; i >= 0 && board[i][y] != 'X'; i--)  
        if(board[i][y] == '0')  
            return false;  
    for (int i = y; i < n && board[x][i] != 'X'; i++)  
        if (board[x][i] == '0')  
            return false;  
    for (int i = y - 1; i >= 0 && board[x][i] != 'X'; i--)  
        if (board[x][i] == '0')  
            return false;  
	return true;
}
void dfs(int num)
{
	for (int i=0;i<n;i++)
	{
		for (int j=0;j<n;j++)
		{
			if (board[i][j]=='.'&&isok(i,j))
			{
				board[i][j]='0';
				dfs(num+1);
				board[i][j]='.';
			}
		}
	}
	if (num>Max) Max=num;
}
int main()
{
	while (cin>>n&&n)
	{
		for (int i=0;i<n;i++)
			cin>>board[i];
		Max=0;
		dfs(0);
		cout<<Max<<endl;
	}
	return 0;
}

回溯法还是不熟练，参考了一下网上别的童鞋的代码才AC的，要多加练习！