# LeetCode OJ - Longest Consecutive Sequence

```1 class LNode{
2     int ele;
3     LNode next;
4     public LNode(int _ele){
5         ele = _ele;
6     }
7 }```

``` 1 /**
2      * Longest Consecutive Sequence
3      * Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
4      * O(n)
5      * @param num
6      * @return
7      */
8     public int longestConsecutive(int[] num){
9         //the most important data structure;
10         HashMap<Integer,LNode> kv = new HashMap<Integer,LNode>(num.length);
11         int max = 0;
12         for(int n: num){
13             if(!kv.containsKey(n)){
14                 LNode l = new LNode(n);
15                 if(kv.containsKey(n-1))
16                     kv.get(n-1).next = l;
17                 if(kv.containsKey(n+1))
18                     l.next = kv.get(n+1);
19                 kv.put(n, l);
20             }
21         }
22         Iterator<LNode> it =  kv.values().iterator();
23         //k is used to keep tracking those node who has been caculated in other sequences
24         //it is to reduce the time complexity, to insure the O(n) time complexity
25         HashMap<Integer,LNode> k = new HashMap<Integer,LNode>();
26
27         while(it.hasNext())
28         {
29             LNode temp = it.next();
30             if(!k.containsKey(temp.ele))
31             {
32                 int nu = 1;
33                 while(temp.next!=null){
34                     temp = temp.next;
35                     k.put(temp.ele, temp);
36                     //kv.remove(temp.ele);
37                     nu++;
38                 }
39                 if(nu>max)
40                     max = nu;
41             }
42         }
43         return max;
44     }```

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LeetCode OJ - Longest Consecutive Sequence

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