leet-longest consecutive sequence length(最长连续序列长度)

最新推荐文章于 2025-09-07 18:41:56 发布

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最新推荐文章于 2025-09-07 18:41:56 发布

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分类专栏： LeetCode 文章标签： c++ LeetCode

本文链接：https://blog.youkuaiyun.com/HeYingShu/article/details/40264173

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本文探讨了如何在未排序的整数数组中找到最长连续整数序列，并提供了高效的O(n)复杂度算法实现。

一概述

Given an unsorted array of itegers,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.

二分析

如果允许O(nLogn)的复杂度，那么可以先排序，但本题要求O(n).

另外，序列里面的元素是无序的，考虑到使用hash表。

用一个哈希表unordered_map<int,bool> used记录每个元素是否使用，对每个元素，以该元素为中心，往左右扩张，直到不连续为止，记录下最长的长度。

三代码

[main.cpp]

#include <iostream>
#include <unordered_map>
#include <vector>
using namespace std;

class Solution{
public:
	int longestConsecutive(const vector<int> &num){
		unordered_map<int,bool> used;
		
		for(vector<int>::const_iterator numIterator = num.begin();numIterator != num.end();numIterator++)
			used[*numIterator] = false;
			
		int longest = 0;
		
		for(vector<int>::const_iterator numIterator = num.begin();numIterator != num.end();numIterator++){
			if(used[*numIterator])
				continue;
			
			int length = 1;
			used[*numIterator] = true;
			int i = *numIterator;
			for(int j = i+1;used.find(j) != used.end();++j){
				used[j] = true;
				++length;
			}
			
			for(int j = i-1;used.find(j) != used.end();--j){
				used[j] = true;
				++length;
			}
			longest = max(longest,length);
		}
		return longest;
	}
};

int main(int argc, char **argv)
{
	Solution *s = new Solution();
	int arr[] = {100,4,200,5,9,2,1,3};
	vector<int> vectorInt(arr,arr+sizeof(arr)/sizeof(int));
	int longestConsecutiveLength = s -> longestConsecutive(vectorInt);
	cout<<"longest consecutive sequence length of arr:"<<endl;
	for(auto i :vectorInt)
		cout<<i<<"\t";
	cout<<"\nis "<<longestConsecutiveLength<<endl;
	return 0;
}

四编译及运算结果

编译:

g++ main.cpp -o longestConsecutiveSequence -std=c++11 -O2

其中:-std=c++11表示启用C++11标准

运算结果:

longest consecutive sequence length of arr:

100 4 200 5 9 2 1 3

is 5