说明：  BP神经网络解决异或问题，改代码注释了自己的编程体会，使用了最少的隐含层神经元解决异或问题，很适合新手对BP神经网络的理解
(BP Neural Network solves XOR problem, changes code to annotate own programming experience, uses the least hidden layer neurons to solve XOR problem, which is very suitable for novice's understanding of BP Neural Network.)