128. 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.

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public class Solution {
    public int longestConsecutive(int[] num) {
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        int max = 1;
        for (int i : num) {
            if (map.containsKey(i)) continue;
            map.put(i, 1);
            if (map.containsKey(i - 1)) {
                max = Math.max(max, merge(map, i-1, i));
            }
            if (map.containsKey(i + 1)) {
                max = Math.max(max, merge(map, i, i+1));
            }
        }
        return max;
    }

    private int merge(HashMap<Integer, Integer> map, int left, int right) {
        int upper = right + map.get(right) - 1;
        int lower = left - map.get(left) + 1;
        int len = upper - lower + 1;
        map.put(upper, len);
        map.put(lower, len);
        return len;
    }
        
    
}