leetcode 268. Missing Number

1. 

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

2. 

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int sum=0,n=nums.size();
        for(int &i:nums) sum+=i;
        return (n*(n+1)/2-sum);
    }
};