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

用異或的辦法：

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;
    }
};

【easy】268. Missing Number