Leetcode 128 longest continuous sequence

subject ： Given an array of unsorted integers , Find out the length of the longest continuous sequence .

The time complexity of the algorithm is required to be  O(n).

Example :

 Input : [100, 4, 200, 1, 3, 2]
 Output : 4
 explain :  The longest continuous sequence is  [1, 2, 3, 4]. Its length is  4.

My solution ：

class Solution {
public:
    int max(int a,int b)
    {
        return a>b?a:b;
    };
   int longestConsecutive(vector<int>& nums) {
       int size=nums.size();
       if(size==0)
         return 0;

         sort(nums.begin(),nums.end());
         int count=1,large=1;
         for(int i=1;i<size;i++)
         {
             if((nums[i]-nums[i-1])==1)
                {count++;}
             else
               {
                   large=max(large,count);
                   count=1;
               }
         }
            return large;
    }
};

Correct solution ：

class Solution {
public:
    int longestConsecutive(vector<int>& nums) {
        int mmax = 0;
        for (int i = 0; i < nums.size(); i ++) 
            if (find(nums.begin(), nums.end(), nums[i] - 1) == nums.end()) { // Not found 
                int cnt = 1, num = nums[i];
                while(find(nums.begin(), nums.end(), num + 1) != nums.end()) // eureka 
                    cnt ++, num ++;
                mmax = max(mmax, cnt);
            }
        return mmax;
    }
};

  Compilation result ：

I haven't figured out why mine is wrong .