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.

 

public class Solution {
    public int longestConsecutive(int[] nums) {
        if (nums == null || nums.length == 0) {
        	return 0;
        }
        HashSet<Integer> hashSet = new HashSet<Integer>();
        for (int i = 0; i < nums.length; i++) {
        	hashSet.add(nums[i]);
		}
        int res = 0;
        for (int i = 0; i < nums.length; i++) {
			if (hashSet.contains(nums[i])) {
				int cnt = 1;
				hashSet.remove(nums[i]);
				int low = nums[i] - 1;
				while (hashSet.contains(low)) {
					cnt++;
					hashSet.remove(low);
					low--;
				}
				int high = nums[i] + 1;
				while (hashSet.contains(high)) {
					cnt++;
					hashSet.remove(high);
					high++;
				}
				res = Math.max(res, cnt);
			}
		}
        return res;
    }
}