2.1.6 Longest Consecutive Sequence

Link: https://oj.leetcode.com/problems/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.

我的思路：雖然知道“遇到不能排序又要O(n)的有序問題，只能增加空間複雜度，用hashset/hashtable. 

但我不知道如何構造這個hashtable。key和value是啥？

答案：hashset即可，不需要hashtable。思路來源於，手動做這道題，看到了4，也想到要去找3和5。

Time: O(n), Space: O(n)

public class Solution {
    public int longestConsecutive(int[] num) {
        Set<Integer> set = new HashSet<Integer>();
        for(int i = 0; i < num.length; i++){
            set.add(num[i]);
        }
        int max = 1;
        while(!set.isEmpty()){
            Iterator it = set.iterator();
            int item = (Integer)it.next();
            int count = 1;
            set.remove(item);
            int left = item - 1;
            while(set.contains(left)){
                count ++;
                set.remove(left);
                left--;
            }
            int right = item + 1;
            while(set.contains(right)){
                count ++;
                set.remove(right);
                right++;
            }
            max = Math.max(max, count);
        }
        return max;
    }
}

Ref: 

http://blog.csdn.net/linhuanmars/article/details/22964467

http://blog.csdn.net/fightforyourdream/article/details/15024861