Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.
Example 1
Input: [3,0,1] Output: 2
Example 2
Input: [9,6,4,2,3,5,7,0,1] Output: 8
用异或的办法：
class Solution { public: int missingNumber(vector<int>& nums) { int len = nums.size(); if(len == 0) return 0; int x = 0; for (int i=1;i<=len;i++) x ^= i; for (int i=0;i<len;i++) x ^= nums[i]; return x; } };
