[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.

将所有element放入set，每check一个element，便找出其所在的最长sequence。并将sequence从set中移除，以免多次check

public class Solution {
public int longestConsecutive(int[] nums) {
Set<Integer> set = new HashSet<>();
int maxlength = 0;
for(int num : nums)
set.add(num);

for(int num : nums){
int length = 1;
int left = num - 1;
int right = num + 1;
while(set.contains(left)){
length++;
set.remove(left);
left--;
}

while(set.contains(right)){
length++;
set.remove(right);
right++;
}

maxlength = Math.max(length, maxlength);
}

return maxlength;
}
}