Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.
For example, Given nums = [0, 1, 3] return 2.
Note: Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?
^：按位异或运算符，是双目运算符。 其功能是参与运算的两数各对应的二进位相异或，当两对应的二进位相异时，结果为1。
特别地，^支持交换律
1^1^2^2^3^3 == 1^2^2^3^3^1 == 1^3^2^3^1^2