符号三角回溯

#include<stdio.h>
int n;
int p[1000][1000];
int sum,half,count,k;
void dfs(int k){
	if(k>n){
		if(count==half){
			sum++;
		}
		return ;
		
	}
	else{
		for(int i=0;i<=1;i++){
			p[1][k]=i;
			count+=i;
			for(int j=2;j<=k;j++){
				p[j][k-j+1]=p[j-1][k-j+1]^p[j-1][k-j+2];
				count+=p[j][k-j+1];		
			}
			if(count<=half&&(k+1)*k/2-count<=half){
					dfs(k+1);
			}	
			for(int j=2;j<=k;j++){
				count-=p[j][k-j+1];	
			}
			count-=i;
		}
	}
}
int main(){
	scanf("%d",&n);
	half=n*(n+1)/4;
	dfs(1);
	printf("%d",sum);
}