[Leetcode] Longest consecutive sequence

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 shold run in O (N) Complexity.

[Note]: There are several pitfalls in this question:

Trap 1: The array elements may be repeated, for example,-, and 2. The result is 4. Your sister's

Pit2: negative numbers are allowed. This should not be considered.

Pit3: The large number may be integer. min_value. Therefore, when sorting by count, consider overflow.

 

Algorithm ideas:

Train of Thought 1: Count sorting; linear sorting algorithm, and then find the longest continuous sequence length.

No bug-free is implemented, so no post is made.

Idea 2: divergence method. If you encounter a number, you want to scatter the two ends and maintain the maximum length.

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

It can also be implemented using recursion. Big Data efficiency is a little lower, and it is no longer implemented. The time and space complexity are O (n ).