NYOJ488 素数环【回溯】+【预处理】

时间：2014-04-16 14:32:18 阅读：455 评论：0 收藏：0 [点我收藏+]

这题要先对n进行奇偶数判断，如果是奇数且不为1则NO answer，否则有解，回溯加剪枝解决。

#include <stdio.h>

int n, store[21], Prime[] = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43 };
bool hasSelect[21], ok;

bool checkPrime(int k){
	for(int i = 0; i < 14; ++i)
		if(k == Prime[i]) return 1;
	return 0;
}

void backTrack(int k){
	if(k == n + 1){
		if(checkPrime(store[1] + store[n])){
			printf("%d", store[1]);
			for(int i = 2; i <= n; ++i) printf(" %d", store[i]);
			printf("\n");
			ok = 1;
		}
		return;
	}
	for(int i = 2; i <= n; ++i){
		if(!hasSelect[i] && checkPrime(i + store[k - 1])){
			hasSelect[i] = 1;
			store[k] = i;
			backTrack(k + 1);
			hasSelect[i] = 0;
		}
	}
}

int main(){
	int i = 1;
	while(scanf("%d", &n), n){
		printf("Case %d:\n", i++);
		if(n & 1){
			if(n == 1) printf("1\n");
			else printf("No Answer\n");
			continue;
		}
		ok = 0;
		store[1] = 1;
		backTrack(2);	
		if(!ok) printf("No Answer\n");
	}
	return 0;
}

815818

长木

素数环