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.

这个方法依赖hashSet，空间复杂度O(N)

Solution：

public class Solution {
    public int longestConsecutive(int[] num) {
        HashSet<Integer> set = new HashSet<Integer>();
        int length = 0;
        for(int e : num){
            set.add(e);
        }
        for(int i = 0; i < num.length;i++){
            int l = num[i] -1;
            int r = num[i] +1;
            int c = 1;
            while(set.contains(l)){
                c++;
                set.remove(l);
                l--;
            }
            while(set.contains(r)){
                c++;
                set.remove(r);
                r++;
            }
            length = Math.max(length,c);
        }
        return length;
    }
}