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.

 

由于算法复杂度是O(n) ，所以不能直接排序。可以把数组存入hashset,考察每个元素的相邻元素是否在set内，把考察过的元素删除。

 

public class Solution {
    public int longestConsecutive(int[] num) {
            HashSet<Integer> hashSet;
        
            if (num.length==0) {
                return 0;
            }
            if (num.length==1) {
                return 1;
            }
            hashSet=new HashSet<>();
            
            for (int i = 0; i < num.length; i++) {
                hashSet.add(num[i]);
            }
            int res=1;
            for (int i = 0; i < num.length; i++) {
                int tmplen=1;
                int tmpnum=num[i]+1;
                while (hashSet.contains(tmpnum)) {
                    tmplen++;
                    hashSet.remove(tmpnum);
                    tmpnum++;
                }
                tmpnum=num[i]-1;
                while (hashSet.contains(tmpnum)) {
                    tmplen++;
                    hashSet.remove(tmpnum);
                    tmpnum--;
                }
                if (tmplen>res) {
                    res=tmplen;
                }
                
            }
            return res;
    }
}