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：如果不对复杂度有要求，可以考虑排序。然而此题不允许超过O（N）复杂度，所以要空间换时间。
因为题中是查找连续元素，所以快速的查找考虑哈希，而且连续元素之间值相差为1，所以据此可以在hashset中逐个元素搜索，找到最大连续子串长度。
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;
}
}