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.

 1     int longestConsecutive(vector<int> &num) {
 2         unordered_map<int, int> num_idx;
 3         for(int i = 0; i < num.size(); i ++) {
 4             num_idx[num[i]] = i;
 5         }
 6         
 7         vector<bool> visited;
 8         for(int i = 0; i < num.size(); i ++) {
 9             visited.push_back(false);
10         }
11         
12         int longest_len = 0;
13         for(int i = 0; i < num.size(); i ++) {
14             if(visited[i] == true)
15                 continue;
16                 
17             int tmp_len = 1;
18             visited[i] = true;
19             //increase
20             int value = num[i] + 1;
21             while(num_idx.find(value) != num_idx.end()){
22                 int index = num_idx[value];
23                 value ++;
24                 visited[index] = true;
25                 tmp_len ++;
26             }
27             //decrease
28             value = num[i] - 1;
29             while(num_idx.find(value) != num_idx.end()){
30                 int index = num_idx[value];
31                 value --;
32                 visited[index] = true;
33                 tmp_len ++;
34             }
35             
36             if(tmp_len > longest_len){
37                 longest_len = tmp_len;
38                 if(longest_len > num.size() / 2)
39                     break;
40             }
41         }
42         
43         return longest_len;
44     }