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！