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题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1016
原题：
A ring is compose of n circles as shown in diagram. Put natural number 1, 2, ..., n into each circle separately, and the sum of numbers in two adjacent circles should be a prime.
Note: the number of first circle should always be 1.
输入N个数字，求1到N这N个数字连成环以后两两相加均为素数的所有情况。
非常经典的DFS题。
#include <iostream> #include <cstring> #include <cstdio> #include <cmath> using namespace std; int n; int cir[21]; int prime[41]; int vis[21]; int isPrime(int x) { int i; int sqx = sqrt((float)x); for(i = 2; i <= sqx; i++) { if(x % i == 0) { return 0; } } return 1; } void dfs(int x) { int i; if(x == n && prime[cir[1]+cir[n]]) //make a circle { for(i = 1; i < n; i++) { printf("%d ", cir[i]); } printf("%d\n", cir[n]); return ; } for(i = 2; i <= n; i++) { if(vis[i] == 0 && prime[cir[x]+i]) { cir[x+1] = i; vis[i] = 1; dfs(x+1); vis[i] = 0; //reset } } } int main() { int i, Case = 1; for(i = 1; i < 41; i++) //list primes. { if(isPrime(i)) { prime[i] = 1; } } while(scanf("%d", &n) != EOF && n) { printf("Case %d:\n", Case++); memset(vis, 0, sizeof(vis)); cir[1] = 1; vis[1] = 1; dfs(1); printf("\n"); } return 0; }
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原文地址：http://www.cnblogs.com/vencentX/p/4515941.html
