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.

Solution:

public class Solution {
    public int longestConsecutive(int[] nums) {
        Map<Integer,Integer> map = new HashMap<>();
        int max = 0;
        for(int n : nums){
            if(!map.containsKey(n)){
                int left = map.containsKey(n-1) ? map.get(n-1) : 0;
                int right = map.containsKey(n+1) ? map.get(n+1) : 0;
                int sum = left+right+1;
                map.put(n, sum);
                max = Math.max(max, sum);
                map.put(n-left, sum);
                map.put(n+right, sum);
            }
        }
        return max;
    }
}