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     }