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 = 0; i < num.size(); i ++) {
            num_idx[num[i]] = i;
        }
        
        vector<bool> visited;
        for(int i = 0; i < num.size(); i ++) {
            visited.push_back(false);
        }
        
        int longest_len = 0;
        for(int i = 0; i < num.size(); i ++) {
            if(visited[i] == true)
                continue;
                
            int tmp_len = 1;
            visited[i] = true;
            //increase
            int value = num[i] + 1;
            while(num_idx.find(value) != num_idx.end()){
                int index = num_idx[value];
                value ++;
                visited[index] = true;
                tmp_len ++;
            }
            //decrease
            value = num[i] - 1;
            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() / 2)
                    break;
            }
        }
        
        return longest_len;
    }