Given a positive integer n, find the least number of perfect square numbers (for example,1, 4, 9, 16, ...) which sum to n.
Given n = 12, return 3 because 12 = 4 + 4 + 4
Given n = 13, return 2 because 13 = 4 + 9
刷这道题目发现网上有四种解法
http://www.cnblogs.com/grandyang/p/4800552.html
可以参照大神的笔记