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.
n (0 < n < 20).
The output format is shown as sample below. Each row represents a series of circle numbers in the ring beginning from 1 clockwisely and anticlockwisely. The order of numbers must satisfy the above requirements. Print solutions in lexicographical order.
You are to write a program that completes above process.
Print a blank line after each case.
Case 1:
1 4 3 2 5 6
1 6 5 2 3 4
Case 2:
1 2 3 8 5 6 7 4
1 2 5 8 3 4 7 6
1 4 7 6 5 8 3 2
1 6 7 4 3 8 5 2
题解：一个素数环问题，dfs去遍历所有可能，为了避免重复，把1固定当做第一个数字。前n-1个数字这样判断：1-n循环，如果没有使用过，并且和前一个数字之和是素数，就填进去。最后一个数字，扫描vis数组，哪个没用过就填进去，判断与前后数字是否和为素数，是则输出数组。素数判定可以打表，到40即可。第一发WA，吓死我了==|，仔细一看原来是case没有，注意空行。
