USACO 1.5.4 Checker Challenge

题意：经典的八皇后问题

解法：

采用朴素的每一次放置都与前面的所有行进行比较，在N =13的时候时间会爆掉

《入门经典》上提供的方法很经典，vis数组的使用，具体见《入门经典》125页

/*
ID:lsswxr1
PROG:checker
LANG:C++
*/
#include<iostream>
#include<fstream>
#include<cmath>
#include<cstring>
using namespace std;
#define maxn 15
ifstream fin("checker.in");
ofstream fout("checker.out");
int N, tot;
int outputNums;
int curPuts[maxn];
int vis[30][30];
void dfs(int cur)
{if (cur == N){tot++;outputNums++;if (outputNums > 3){return;}for (int i = 0; i < N; i++){if (i == 0){fout << curPuts[i] + 1;}else{fout << " " << curPuts[i] + 1;}}fout << endl;return;}for (int i = 0; i < N; i++){if (!vis[0][i] && !vis[1][cur + i] && !vis[2][cur - i + N]){curPuts[cur] = i;vis[0][i] = vis[1][cur + i] = vis[2][cur - i + N] = 1;dfs(cur + 1);vis[0][i] = vis[1][cur + i] = vis[2][cur - i + N] = 0;}}}int main()
{fin >> N;tot = 0;memset(vis, 0, sizeof(vis));memset(curPuts, 0, sizeof(curPuts));dfs(0);fout << tot << endl;//cout<<ans<<endl;return 0;
}