[LeetCode] Longest Consecutive Sequence

原题链接： 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.

思路：首先想到的是排序，显然排序O(nlogn)，然后数组扫描O(n)。复杂度不符合题目要求。

O(n)的算法如果没有巧妙的解法基本都是空间换时间，用HashSet存入每个数字，每个数字只会出现在一个字串中，因此拿出Set中一个数字，每次加 1看是否在HashSet中，是则计数加1。同理减1也一样，每次判断完后都从Set中删除该数字。总共也是需要O(n)的时间。

public class Solution {
    public int longestConsecutive(int[] num) {
        if (num == null || num.length < 1) {
            return 0;    
        }
        
        HashSet<Integer> numSet = new HashSet<Integer>();
        
        for (int a : num) {
            numSet.add(a);
        }
        
        int maxCnt = 0;
        while (!numSet.isEmpty()) {
            Iterator<Integer> it = numSet.iterator();
            Integer curNum = it.next();
            it.remove();
            int count = 1;
            
            int bigger = curNum;
            while (numSet.contains(++bigger)) {
                count++;
                numSet.remove(bigger);
            }
            
            int litter = curNum;
            while (numSet.contains(--litter)) {
                count++;
                numSet.remove(litter);
            }
            
            if (count > maxCnt) {
                maxCnt = count;
            }
        }
        
        return maxCnt;
    }
}