Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Your algorithm should run in O(n) complexity.

Example:

Input: [100, 4, 200, 1, 3, 2]
Output: 4
Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.

思路：如果不對複雜度有要求，可以考慮排序。然而此題不允許超過O（N）複雜度，所以要空間換時間。

因為題中是查詢連續元素，所以快速的查詢考慮雜湊，而且連續元素之間值相差為1，所以據此可以在hashset中逐個元素搜尋，找到最大連續子串長度。

class Solution {
 
    public int longestConsecutive(int[] nums) {
        Set<Integer> set = new HashSet<Integer>();
        for(int i :nums){
            set.add(i);
        }
        int ans = 0;
        for(int i : set){
            if(set.contains(i-1)) continue;
            int now = i;
            int sum = 1;
            while(set.contains(now+1)){
                now++;
                sum++;
            }
            ans = Math.max(ans,sum);
        }
        return ans;
    }
}