It use the hashset to do the tricks.

1 class Solution { 2 public: 3     int longestConsecutive(vector
     
       &num) { 4         int len = num.size(), result = 0; 5         if (len < 2) return len; 6         unordered_set
      
        sets; 7         for (int i = 0; i < len; i++) { 8             sets.insert(num[i]); 9         }10         while (!sets.empty()) {11             int start = *sets.begin(), end = start;12             while (sets.erase(start-1)) start--;13             while (sets.erase(end)) end++;14             result = max(result, end-start);15         }16         return result;17     }18 };