将1,2，3，。。。,20这20个连续的自然数排成一圈，使任意两个相邻的自然数之和均为素数。

    网上看到很多地方有这个题，但是好像没有多少人用C++实现，今天有时间实现了一下并进行了扩展，N可以为任意的自然数，并不限制于20，供大家参考一下~

#include "iostream" #include "math.h" #define N 24 //N maybe any number using namespace std; int isPrime(int num); int main() { int num[N]; //original data from 1 to N int arr[N]; //the constructed prime cycle int index[N]; //the search index of each line int i,j; arr[0] = N; for (i=0;i<N;i++) { num[i] = i+1; if (arr[0]%2==0)//the last number of num[N] is even { if (i%2==0) { index[i] = 0; } else index[i] = 1; } else//the last number of num[N] is old { if (i%2==0) { index[i] = 1; } else index[i] = 0; } } for (i=0;i<N;i++) { j = index[i]; for (;j<N;j=j+2) { if (num[j]!=0 && isPrime(arr[i]+num[j])) { if (i<N-2) { arr[i+1] = num[j]; num[j] = 0;//assignment the picked number of 0