Longest Consecutive Sequence leetcode java

题目：

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.

题解：

这道题利用HashSet的唯一性解决，能使时间复杂度达到O(n)。首先先把所有num值放入HashSet，然后遍历整个数组，如果HashSet中存在该值，就先向下找到边界，找的同时把找到的值一个一个从set中删去，然后再向上找边界，同样要把找到的值都从set中删掉。所以每个元素最多会被遍历两边，时间复杂度为O(n)。

代码如下：

 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          
         int max = 0;  
         for(int i=0; i<num.length; i++){  
             if(hs.contains(num[i])){
                 int count = 1;  
                 hs.remove(num[i]);
                 
                 int low = num[i] - 1; 
                 while(hs.contains(low)){  
                     hs.remove(low);  
                     low--;  
                     count++;  
                 }
                 
                 int high = num[i] + 1;  
                 while(hs.contains(high)){  
                     hs.remove(high);  
                     high++;  
                     count++;  
                 }  
                 max = Math.max(max, count);  
             }  
         }  
         return max;  
     }