【LeetCode】Longest Consecutive Sequence

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.

Discuss
题目要求O(N)的时间复杂度，显然不能排序，考虑哈希，从某个数开始递增递减查询，更新最长长度，这里需要考虑一些退化的情况：
当输入是形如 【1,2,3,4,5.......n】的已经排好序的连续输入时候，算法可能退化成O（n^2），故考虑在哈希查询到的时候一并删除该元素，并在后面加判断长度是否已经
大于等于数组长度，java code :

public class Solution {
public int longestConsecutive(int[] num) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
if(num.length == 0)
return 0;
if(num.length == 1)
return 1;
HashSet<Integer> hash = new HashSet<Integer>();
for(int i = 0; i < num.length; i++)
{
hash.add(num[i]);
}
int res = 0;
for(int i = 0; i < num.length; i++)
{
int tmplen = 1, tmpnum;
tmpnum = num[i] + 1;
while(hash.contains(tmpnum))
{
hash.remove(tmpnum);
tmplen++;
tmpnum++;
}
tmpnum = num[i] - 1;
while(hash.contains(tmpnum))
{
hash.remove(tmpnum);
tmplen++;
tmpnum--;
}
if(tmplen > res)
res = tmplen;
if(res >= num.length)
break;
}
return res;
}
}