Algorithm Gossip :八皇后问题

/*八皇后问题。*/

#include <stdio.h>
#include <stdlib.h>

#define N 8

int column[N+1];//同栏是否有皇后，1表示有
int rup[2*N+1];//右上至左下是否有皇后
int lup[2*N+1];//左上至右下是否有皇后
int queen[N+1] = {0};
int num;//解答编号

void backtrack(int);//递回求解

int main(void)
{
int i;
num = 0;
for( i = 1;i <= N;i++ )
{
column[i] = 1;
}
for( i = 1;i <= 2*N;i++ )
{
rup[i] = lup[i] = 1;
}
backtrack(1);
return 0;
}

void showAnswer()
{
int x,y;
printf("\nanswer %d\n",++num);
for( y = 1;y <= N;y++ )
{
for( x = 1;x <= N;x++ )
{
if(queen[y] == x)
{
printf("Q");
}
else
{
printf(".");
}
}
printf("\n");
}

}

void backtrack(int i)
{
int j;
if( i > N )
{
showAnswer();
}
else
{
for( j = 1;j <= N;j++ )
{
if(column[j] == 1 &&
rup[i+j] == 1 &&
lup[i-j+N] == 1)
{
queen[i] = j;
column[j] = rup[i+j] = lup[i-j+N] = 0;
backtrack(i+1);
column[j] = rup[i+j] = lup[i-j+N] = 1;
}
}
}
}