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，找出其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，去除了重复。同时，对数组中的每一个元素，检测其左右侧元素(+-1)是否在set集合中。

	public static int longestConsecutive(int[] num) {
		Set<Integer> set = new HashSet<Integer>();
		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;
	}