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?

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int len = nums.size();
        if (len==1) {
            return nums[0]==1?0:1;
        }
        sort(nums.begin(), nums.end());
        if (nums[0]!=0) {
            return 0;
        }
        for (int i=1; i<len; i++) {
            if (i==0&&nums[i]!=0) {
                return 0;
            }
            if (nums[i]!=nums[i-1]+1) {
                return nums[i]-1;
            }
        }
        return nums[len-1]+1;
    }
};

第二种解法：
```c++
class Solution {
public:
int missingNumber(vector& nums) {
int result = nums.size();
int i=0;

    for(int num:nums){
        result ^= num;
        result ^= i;
        i++;
    }

    return result;
}

};
```