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?

#####思路 0、1、2....n之和expectedSum为(n+1)*n/2

拿掉一个数k后的和sum为(n+1)*n/2-k

因此k = expectedSum - sum