Longest Consecutive Sequence leetcode java

題目：

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.

 

題解：

這道題利用HashSet的唯一性解決，能使時間複雜度達到O(n)。首先先把所有num值放入HashSet，然後遍歷整個陣列，如果HashSet中存在該值，就先向下找到邊界，找的同時把找到的值一個一個從set中刪去，然後再向上找邊界，同樣要把找到的值都從set中刪掉。所以每個元素最多會被遍歷兩邊，時間複雜度為O(n)。

程式碼如下：

 

 1     public int longestConsecutive(int[] num) {  
 2         if(num == null||num.length == 0)
 3             return 0;
 4         
 5         HashSet<Integer> hs = new HashSet<Integer>();  
 6         
 7         for (int i = 0 ;i<num.length; i++)   
 8             hs.add(num[i]);  
 9          
10         int max = 0;  
11         for(int i=0; i<num.length; i++){  
12             if(hs.contains(num[i])){
13                 int count = 1;  
14                 hs.remove(num[i]);
15                 
16                 int low = num[i] - 1; 
17                 while(hs.contains(low)){  
18                     hs.remove(low);  
19                     low--;  
20                     count++;  
21                 }
22                 
23                 int high = num[i] + 1;  
24                 while(hs.contains(high)){  
25                     hs.remove(high);  
26                     high++;  
27                     count++;  
28                 }  
29                 max = Math.max(max, count);  
30             }  
31         }  
32         return max;  
33     }