uva11195(回溯+位运算)

题意:

八皇后;

*的位置不能放;

思路:

直接回溯会超时,

所以用位运算;

AC代码:

#include<cstdio>#include<cstring>#include<algorithm>using namespace std;const int N = 20;int INF;int n, sum;char g[N][N];int s[N];int dfs(int cur, int d, int lu, int ru) {int res = 0;if(cur == n) {return 1;}int p = ~(s[cur] | d | lu | ru);int sta = p & (-p) & INF;while(sta) {res += dfs(cur + 1, d|sta , (sta|lu) << 1 ,(sta|ru) >> 1);p ^= sta;sta = p & (-p) & INF;}return res;}int main() {int cas = 1;while(scanf("%d",&n) && n) {INF = (1 << n) - 1;memset(s, 0, sizeof(s));for(int i = 0; i < n; i++) {scanf("%s",g[i]);for(int j = 0; j < n; j++) {if(g[i][j] == '*')s[i] |= (1 << j);}}printf("Case %d: %d\n",cas++, dfs(0, 0, 0, 0));}}