HDU2553 （N皇后）

题意：N皇后

方法：BFS+打表

#include<stdio.h>
#include<string.h>
const int maxn = 21;
int sd[ maxn*2 ],c[ maxn ],md[ maxn*2 ];
struct node{
    int r,c,flag;
}p,stack[ maxn*maxn ];
int ans,n;

void bfs(){
    int top=-1;
    for( int i=n-1;i>=0;i-- ){
        stack[ ++top ].r=0;
        stack[ top ].c=i;
        stack[ top ].flag=0;//0 stand for not the path
    }
    memset( sd,0,sizeof(sd));
    memset( c,0,sizeof(c));
    memset( md,0,sizeof(md));
    while(  top>=0 ){
        node now;
        now=stack[ top ];
        if( now.flag==0 ){
            if( sd[ now.r+now.c ]==1 || c[ now.c ]==1 || md[ now.r-now.c+n-1 ]==1 ) top--;
            else {
                sd[ now.r+now.c ]=c[ now.c ]=md[ now.r-now.c+n-1 ]=1;
                stack[ top ].flag=1;//标记待会要出栈
                if( now.r==n-1 ) ans++;
                else{
                    for( int i=n-1;i>=0;i--){
                         stack[++top].r=now.r+1;
                         stack[top].c=i;
                         stack[top].flag=0;
                    }//put the next line into the stack
                }
            }
        }
        else{
            sd[ now.r+now.c ]=c[ now.c ]=md[ now.r-now.c+n-1 ]=0;
            top--;
        }
    }
    return ;
}

int main(){
    while( scanf("%d",&n) && n ){
        ans=0;
        bfs();
        printf("%d\n",ans);
    }
    return 0;
}