Single Number

Given an array of integers, every element appears twice except for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

解：

利用位运算中的异或运算，①当两个值相同时，异或后为0；②0与任何值异或均为该值本身

def singleNumber(A):
    if len(A) == 0:
        return
    ret = 0
    for item in A:
        ret ^= item
    return ret