[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.

[Solution].java

import java.util.HashMap;
public class Solution {
    public int longestConsecutive(int[] num) {
        // Start typing your Java solution below
        // DO NOT write main() function
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        for(int i = 0; i < num.length; i++){
            int n = num[i];
            if(map.containsKey(n)) continue;
            if(map.containsKey(n - 1) || map.containsKey(n + 1)){
              int value = 1;
            	int beforeOffset = 0;
            	if(map.containsKey(n - 1)){
            		beforeOffset = map.get(n - 1);
            		value += map.get(n - 1);
            		map.put(n - beforeOffset, value);
            		map.put(n, value);
            	}
            	if(map.containsKey(n + 1)){
            		value += map.get(n + 1);
            		map.put(n + map.get(n + 1), value);
            		map.put(n, value);
            	}
            	if(map.containsKey(n - 1)) map.put(n - beforeOffset, value);
        	}
        	else map.put(n, 1);
        }
        int max = 0;
        for(Integer len: map.values()){
        	if(len > max) max = len;
        }
        return max;
    }
}