LeetCode: Longest Consecutive Sequence

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

  #define tr(c,it) for(decltype((c).begin()) (it) = (c).begin(); (it) != (c).end(); (it)++)


   int longestConsecutive(vector<int> &num) {  
     int res = 1,local = 1;  
     int n = num.size();  
     if(n==0)return 0;  
     sort(num.begin(),num.end());  
     set<int> s;  
     tr(num, it)  
     {  
       s.insert(*it);  
     }  
     num.clear();  
     n = 0;  
     tr(s, it)  
     {  
       num.push_back(*it);  
       n++;  
     }  
     for (int i = 1; i < n; i++)  
     {  
       if(num[i] == num[i-1]+1)  
       {  
         local++;  
         if(local > res)res = local;  
       }  
       else  
         local = 1;  
     }  
     return res;  
   }