Longest Consecutive Sequence [LeetCode]

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.

Summary: The key idea is using hash map to record every number and their index.

     int longestConsecutive(vector<int> &num) {
         unordered_map<int, int> num_idx;
         for(int i = ; i < num.size(); i ++) {
             num_idx[num[i]] = i;
         }

         vector<bool> visited;
         for(int i = ; i < num.size(); i ++) {
             visited.push_back(false);
         }

         int longest_len = ;
         for(int i = ; i < num.size(); i ++) {
             if(visited[i] == true)
                 continue;

             int tmp_len = ;
             visited[i] = true;
             //increase
             int value = num[i] + ;
             while(num_idx.find(value) != num_idx.end()){
                 int index = num_idx[value];
                 value ++;
                 visited[index] = true;
                 tmp_len ++;
             }
             //decrease
             value = num[i] - ;
             while(num_idx.find(value) != num_idx.end()){
                 int index = num_idx[value];
                 value --;
                 visited[index] = true;
                 tmp_len ++;
             }

             if(tmp_len > longest_len){
                 longest_len = tmp_len;
                 if(longest_len > num.size() / )
                     break;
             }
         }

         return longest_len;
     }