【面试题】剑指offer30--最小的k个数

ゝ一世哀愁。 2022-06-12 02:09 192阅读 0赞

输入n个整数，求出最小的k个数。

使用简单的方法，对这组数进行升序排序，可以找到前k个数字

代码实现：

#include<iostream>
    using namespace std;
    
    int PartSort(int *arr, int left, int right)//找到中间位置
    {
    	while (left<right)
    	{
    		while (left<right&&arr[left] <= arr[right])
    		{
    			left++;
    		}
    		while (left<right&&arr[right] >= arr[left])
    		{
    			right--;
    		}
    		std::swap(arr[left], arr[right]);
    	}
    	return left;
    }
    
    void QuickSort(int* arr, int left, int right)
    {
    	if (left < right)
    	{
    		int mid = PartSort(arr, left, right);
    		QuickSort(arr, left, mid - 1);
    		QuickSort(arr, mid + 1, right);
    	}
    }
    void PrintMinNum(int* arr, size_t n)
    {
    	cout << "最小的" << n << "个数是" << endl;
    	for (size_t i = 0; i < n; i++)
    	{
    		cout << arr[i] << " ";
    	}
    	cout << endl;
    }
    void TestFind()
    {
    	int arr[] = { 0, 5, 6, 7, 2, 8, 3, 8, 1 };
    	QuickSort(arr, 0, sizeof(arr) / sizeof(arr[0]) - 1);
    	PrintMinNum(arr, 3);
    }
    
    
    int main()
    {
    	TestFind();
    	system("pause");
    	return 0;
    }

运行的结果:

![Center][]

首先使用快速排序的方法对这组数进行排序，如果排的数字第k个位置，那第k个位置以前的就是最小的k个数了。

代码的实现：

#include<iostream>
    using namespace std;
    
    int PartSort(int *arr, int left, int right)//找到中间位置
    {
    	while (left<right)
    	{
    		while (left<right&&arr[left] <= arr[right])
    		{
    			left++;
    		}
    		while (left<right&&arr[right] >= arr[left])
    		{
    			right--;
    		}
    		std::swap(arr[left], arr[right]);
    	}
    	return left;
    }
    
    void FindMinKNum(int* arr, int left, int right, int index)//快速排序法
    {
    	if (left < right)
    	{
    		int mid = PartSort(arr, left, right);
    		if (mid == (index - 1))
    		{
    			return;
    		}
    		else if (mid>(index - 1))
    		{
    			return FindMinKNum(arr, left, mid-1, index);
    		}
    		else
    		{
    			return FindMinKNum(arr, mid+1, right, index);
    		}
    	}
    }
    void PrintMinNum(int* arr, size_t n)
    {
    	cout << "最小的" << n << "个数是" << endl;
    	for (size_t i = 0; i < n; i++)
    	{
    		cout << arr[i] << " ";
    	}
    	cout << endl;
    }
    void TestFind()
    {
    	int arr[] = { 2, 5, 6, 7, 2, 8, 3, 8, 1 };
    	FindMinKNum(arr, 0, sizeof(arr) / sizeof(arr[0])-1,3);
    	PrintMinNum(arr, 3);
    }
    
    
    int main()
    {
    	TestFind();
    	system("pause");
    	return 0;
    }

运行的结果：

![Center 1][]

第三种方法是使用红黑树的方法来查找，只不过打印出来的数是按照从大到小的顺序的：

代码实现：

#include<iostream>
    using namespace std;
    #include<set>
    struct greater
    {
    	bool operator()(const int& i1, const int& i2)
    	{
    		return i1 > i2;
    	}
    };
    
    pair<bool, multiset<int, greater>>FindMinNum(int* arr,size_t n, int index)
    {
    	if (index < 1)
    	{
    		return pair<bool, multiset<int, greater>>(false, multiset<int, greater>());
    	}
    	multiset<int, greater> Sset;
    	for (size_t i = 0; i < n; ++i)
    	{
    		if (Sset.size() < index)
    		{
    			Sset.insert(arr[i]);
    		}
    		else
    		{
    			if (arr[i] < *Sset.begin())
    			{
    				Sset.erase(Sset.begin());
    				Sset.insert(arr[i]);
    			}
    		}
    	}
    	if (Sset.size() < index)
    	{
    		return pair<bool, multiset<int, greater>>(false, Sset);
    	}
    	return pair<bool, multiset<int, greater>>(true, Sset);
    }
    
    void TestFind()
    {
    	int arr[] = { 1, 4, 3, 7, 8, 2, 9 };
    	int n = 0;
    	cout << "请输入k的值->" << endl;
    	cin >> n;
    	pair<bool, multiset<int, greater>> ret = FindMinNum(arr, sizeof(arr) / sizeof(arr[0]),n);
    	cout << "最小的"<<n<<"个数为：" << endl;
    	if (ret.first)
    	{
    		multiset<int, greater>::iterator it = ret.second.begin();
    		while (it != ret.second.end())
    		{
    			cout << *it << " ";
    			++it;
    		}
    		cout << endl;
    	}
    	else
    	{
    		cout << "数组出错了" << endl;
    	}
    }
    
    int main()
    {
    	TestFind();
    	system("pause");
    	return 0;
    }

运行结果：

![Center 2][]

[Center]: /images/20220612/81a7c274a448415aa714f94cebbd62e7.png
[Center 1]: /images/20220612/d37c905f10534f01b0a4d44901163cce.png
[Center 2]: /images/20220612/21ce16a17fba4c0e872899dec701c4e6.png