Longest Consecutive Sequence

Posted 2020-08-03 北叶青藤

Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

Clarification

Your algorithm should run in O(n) complexity.

Example

Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

分析：

既然要求O(n) complexity，排序这种方法就放弃了。这里用了HashSet来保存每个数，然后从这个数开始，左右寻找是否有相邻的数。非常有新意的方法。

 1 public class Solution {
 2 
 3     public int longestConsecutive(int[] num) {
 4         if (num == null || num.length == 0) return 0;
 5 
 6         HashSet<Integer> hs = new HashSet<Integer>();
 7         for (int i = 0; i < num.length; i++) {
 8             hs.add(num[i]);
 9         }
10 
11         int max = 0;
12         for (int i = 0; i < num.length; i++) {
13             if (hs.contains(num[i])) {
14                 int count = 1;
15                 hs.remove(num[i]);
16 
17                 int low = num[i] - 1;
18                 while (hs.contains(low)) {
19                     hs.remove(low);
20                     low--;
21                     count++;
22                 }
23 
24                 int high = num[i] + 1;
25                 while (hs.contains(high)) {
26                     hs.remove(high);
27                     high++;
28                     count++;
29                 }
30                 max = Math.max(max, count);
31             }
32         }
33         return max;
34     }
35 }

参考请注明出处：cnblogs.com/beiyeqingteng/