LeetCode – Longest Consecutive Sequence Java

最新推荐文章于 2019-09-17 09:24:18 发布

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最新推荐文章于 2019-09-17 09:24:18 发布

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分类专栏：算法文章标签： leetcode

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本文介绍了一种寻找整数数组中最长连续元素序列的算法，并提供了一个Java实现方案。该算法利用HashSet来存储和检查元素，确保操作的时间复杂度为O(n)。

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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.

Thoughts

Because it requires O(n) complexity, we can not solve the problem by sorting the array first. Sorting takes at least O(nlogn) time.

Java Solution 1

We can use a HashSet to add and remove elements. HashSet is implemented by using a hash table. Elements are not ordered. The add, remove and contains methods have constant time complexity O(1).

public int longestConsecutive(int[] num) {
	Set<Integer> set = new HashSet<Integer>();
	int max = 1;
 
	for (int e : num)
		set.add(e);
 
	for (int e : num) {
		int left = e - 1;
		int right = e + 1;
		int count = 1;
 
		while (set.contains(left)) {
			count++;
			set.remove(left);
			left--;
		}
 
		while (set.contains(right)) {
			count++;
			set.remove(right);
			right++;
		}
 
		max = Math.max(count, max);
	}
 
	return max;
}

After an element is checked, it should be removed from the set. Otherwise, time complexity would be O(mn) in which m is the average length of all consecutive sequences.