LeetCode 128. Longest Consecutive Sequence

原题链接在这里：https://leetcode.com/problems/longest-consecutive-sequence/

题目：

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.

题解：

用HashSet把所有点放入hs中.

在从头逐个检查是否包含这个点，若是包含，就看它往下能走多远，然后看往上能走多远，走过的点从hs中移除，维护最大值。

Time Complexity: O(n). Space O(n).

AC Java:

public class Solution {
    public int longestConsecutive(int[] nums) {
        if(nums == null || nums.length == 0){
            return 0;
        }
        HashSet<Integer> set = new HashSet<Integer>();
        for(int num : nums){
            set.add(num);
        }
        
        int res = 0;
        for(int num : nums){
            if(set.contains(num)){
                int count = 1;
                set.remove(num);
                
                //check how many nodes are there in lower branch
                int low = num-1;
                while(set.contains(low)){
                    count++;
                    set.remove(low);
                    low--;
                }
                
                //check how many nodes are there in higher branch
                int high = num+1;
                while(set.contains(high)){
                    count++;
                    set.remove(high);
                    high++;
                }
                
                res = Math.max(res, count);
            }
        }
        return res;
    }
}

 类似Binary Tree Longest Consecutive Sequence.