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.

原题链接：https://oj.leetcode.com/problems/longest-consecutive-sequence/

题目：给定一个未排序的整数数组，找出当中最长的递增元素序列的长度。

你的算法的复杂度为O(N)。

思路：使用HashSet，先将数组中的元素放入。去除了反复。同一时候，对数组中的每个元素。检測其左右側元素(+-1)是否在set集合中。

public static int longestConsecutive(int[] num) {		Set
     
       set = new HashSet
      
       ();		int max = -1;		for (int ele : num)			set.add(ele);		for (int ele : num) {			int left = ele - 1;			int right = ele + 1;			int count = 1;			while (set.contains(left)) {				count++;				set.remove(left);				left--;			}			while (set.contains(right)) {				count++;				set.remove(right);				right++;			}			max = Math.max(count, max);		}		return max;	}