剑指offer-40

最小的K个数，输入N个数，找出最小的K个数

解法一：先排序再数数O（NlogN）

解法二：用最大堆来保存数据，直至遍历完毕，

解法三：用快速排序法找出第K位位置，K位左侧既是最小的K个数

解法一：
void quickSort(int* input,int len, int start, int end)
{
	if (start < 0 || end < 0 || start >= len || end >= len || start >= end) return;
	
	int l = start + 1;
	int h = end;
	while (l<=h)
	{
		while (h>start && input[h]> input[start]) h--;
		while (l < h && input[l] <= input[start]) l++;
		if (l<h)
		{
			int temp = input[l];
			input[l] = input[h];
			input[h] = temp;
		}
		else
			break;
	}

	if (h>start)
	{
		int temp = input[start];
		input[start] = input[h];
		input[h] = temp;
	}
	quickSort(input, len, start, h - 1);
	quickSort(input, len, h + 1, end);
}
void GetLeastNumbers_Solution3(int* input, int n, int* output, int k)
{
	if (input == nullptr || n <= 0 || output == nullptr || k <= 0 || k > n) return;
	
	int* sortArray = new int[n]();
	for (int i = 0; i < n; i++)
		sortArray[i] = input[i];

	quickSort(sortArray, n, 0, n - 1);
	for (int i = 0; i < k;i++)
		output[i] = sortArray[i];
	return;
}

解法二：
void GetLeastNumbers_Solution4(int* input, int n, int* output, int k)//diy heap
{
	if (input == nullptr || n <= 0 || output == nullptr || k <= 0 || k>n) return;

	std::priority_queue<int> smallHeap;
	for (int i=0;i<n;i++)
	{
		if (smallHeap.size()<k)
			smallHeap.push(input[i]);
		else if (smallHeap.size()>=k && input[i]<=smallHeap.top())
		{
			smallHeap.pop();
			smallHeap.push(input[i]);
		}
	}
	for (int i=0;i<k;i++)
	{
		(output)[i] = smallHeap.top();
		smallHeap.pop();
	}
}

解法3：在1的基础上改了一下
void findKNum(int* input, int len,int k, int start, int end)//diy
{
	if (start > k) return;

	int l = start + 1;
	int h = end;
	while (l <= h)
	{
		while (h>start && input[h]> input[start]) h--;
		while (l < h && input[l] <= input[start]) l++;
		if (l<h)
		{
			int temp = input[l];
			input[l] = input[h];
			input[h] = temp;
		}
		else
			break;
	}

	if (h>start)
	{
		int temp = input[start];
		input[start] = input[h];
		input[h] = temp;
	}
	if (h == k)
		return;
	else if (h<k)
		return findKNum(input, len, k, h + 1, end);
	else
		return findKNum(input, len, k, start, h - 1);
}
void GetLeastNumbers_Solution4(int* input, int n, int* output, int k)//diy
{
	if (input == nullptr || n <= 0 || output == nullptr || k <= 0 || k > n) return;

	int* sortArray = new int[n]();
	for (int i = 0; i < n; i++)
		sortArray[i] = input[i];

	findKNum(sortArray, n, k-1, 0, n - 1);
	for (int i = 0; i < k; i++)
		output[i] = sortArray[i];
	return;
}