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?

 1 class Solution { 2 public: 3     int missingNumber(vector<int>& nums) { 4         int number = 0; 5         for(int i = 0; i <= nums.size(); i++) 6             number ^= i; 7         for(auto n : nums) 8             number ^= n; 9             10         return number;11     }12 };

268. Missing Number