素数环问题 　　把从1到20这20个数摆成一个环，要求相邻的两个数的和是一个素数。 分析：用回溯算法，考察所有可能的排列。-Central questions primes from 1-20 to 20 the number of put it into a ring, requiring two adjacent numbers and is a prime number. Analysis: backtracking algorithm to study all the possi