[Note #4][Algorithm] Backtracking

Backtracking: http://acm.nudt.edu.cn/~twcourse/Backtracking.html

Example:

• Problem: 
• A ring is composed of n (even number) circles as shown in diagram. Put natural numbers 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.
• You are to write a program that completes above process.
• AC: https://github.com/Meng-Gen/UVa/blob/master/UVa/acm_524.cc