原题链接在这里：

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：

用HashSet把所有点放入hs中.

在从头逐个检查是否包含这个点，若是包含，就看它往下能走多远，然后看往上能走多远，走过的点从hs中移除，维护最大值。

Time Complexity: O(n). Space O(n).

AC Java:

1 public class Solution { 2     public int longestConsecutive(int[] nums) { 3         if(nums == null || nums.length == 0){ 4             return 0; 5         } 6         HashSet
     
       set = new HashSet
      
       (); 7         for(int num : nums){ 8             set.add(num); 9         }10         11         int res = 0;12         for(int num : nums){13             if(set.contains(num)){14                 int count = 1;15                 set.remove(num);16                 17                 //check how many nodes are there in lower branch18                 int low = num-1;19                 while(set.contains(low)){20                     count++;21                     set.remove(low);22                     low--;23                 }24                 25                 //check how many nodes are there in higher branch26                 int high = num+1;27                 while(set.contains(high)){28                     count++;29                     set.remove(high);30                     high++;31                 }32                 33                 res = Math.max(res, count);34             }35         }36         return res;37     }38 }
      
     

 类似.