[leetcode-128]Longest Consecutive Sequence(java)

问题描述：
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.

分析：这道题最直接的想法是排序，然后再进行一次遍历，但是时间复杂度将变成O(nlogn)。那么可以想到是不是可以使用hash集合，这样查找的时间为O(1)。为了做到这点，我们先将每个元素添加到hash集合中。然后再遍历每个元素，在遍历每个元素时，分别向两边扩展。当找不到相邻元素时退出。

代码如下：380ms

public class Solution {    public int longestConsecutive(int[] nums) {        HashSet<Integer> sets = new HashSet<>();        for(int i = 0;i<nums.length;i++)            sets.add(nums[i]);        int max = 0;        int count;        for(int i = 0;i<nums.length;i++){            count = 0;            int val = nums[i];            int valTmp = val;            while(sets.contains(valTmp)){                count++;                sets.remove(val);                valTmp++;            }            while(sets.contains(val-1)){                count++;                sets.remove(val-1);                val--;            }            if(count>max)                max = count;        }        return max;    }}