【剑指 Offer 41. 数据流中的中位数】+手撕堆排代码解析+类成员变量作引用传参解析

害怕面试官不允许使用库，一怒之下手撕该题代码，手撕堆排（heapSort），make_heap/pop_heap/push_heap函数；另外因为要实现堆排，涉及到了类内成员变量作引用传参的知识点，这个应该面试也会问；下面上代码和问题

1. 类内没有实例化模板类，报错C2440,“初始化”: 无法从“initializer list”转换为，必须实例化模板类。
2. 类内成员变量作引用，必须声明 为引用变量，同时采用有参构造的初始化形式进行初始化。否则无法对外部变量进行更改；直接赋值，报错您没有初始化。

#include 
#include 
#include 
using namespace std;

struct myless { bool operator ()(int a, int b) { return a < b; } };
struct mygreater { bool operator ()(int a, int b) { return a > b; } };
template
class Heap {
public:
	a cmp;//实例化模板类，不然报错C2440
	vector & heap; //必须声明为引用变量，不然无法对外部变量进行更改。
	Heap() {};//无参构造
	Heap(vector  &vt) :heap(vt) {
		//heap = vt;//赋值报错，必须使用初始化形式 :heap(vt)
	};//有参构造
	void heapSort() {//O(nlogn)
		heapBuild();
		for (int i = heap.size() - 1; i > 0; i--) {
			swap(heap[0], heap[i]);
			heapAdjust(heap, 0, i - 1);
		}
	}
	void heapBuild() {//O(logn)
		for (int i = (heap.size() - 2) / 2; i >= 0; i--) {
			heapAdjust(heap, i, heap.size() - 1);
		}
	}
	void heapAdjust(vector &heap, int s, int m) {
		int temp, j;
		temp = heap[s];
		for (j = 2 * s + 1; j <= m; j = j * 2 + 1) {
			if (j < m && cmp(heap[j], heap[j + 1])) j++;
			if (!cmp(temp, heap[j])) break;
			heap[s] = heap[j];
			s = j;
		}
		heap[s] = temp;
	}
	void popHeap() {//O(logn)
		swap(heap.front(), heap.back());
		heapAdjust(heap, 0, heap.size() - 2);
		heap.pop_back();
	}
	void pushHeap(int num) {//O(logn)
		heap.push_back(num);
		int s = heap.size() - 1;
		int r = (heap.size() - 2) / 2;//获取s的根节点索引
		while (r >= 0 && cmp(heap[r], heap[s])) {//如果后进来的s 大于/小于 r，进行置换，并向上一层递进
			swap(heap[r], heap[s]);
			s = r; r = (s - 1) / 2; //递进
		}
	}
};

int main() {
	vector vtTest = { 50,10,90,30,70,40,80,60,20 };
	Heap bigheap(vtTest);
	bigheap.heapBuild();
	for_each(vtTest.begin(), vtTest.end(), [](int num) {cout << num << " "; }); cout << endl;
	bigheap.popHeap();
	for_each(vtTest.begin(), vtTest.end(), [](int num) {cout  << num<< " "; }); cout << endl;

	cout << "--------------大堆下排序-------------" << endl;
	bigheap.heapSort();
	for_each(vtTest.begin(), vtTest.end(), [](int num) {cout << num << " "; }); cout << endl;

	cout << "--------------实现小堆下排序-------------" << endl;
	Heap smallheap(vtTest);
	smallheap.heapSort();
	for_each(vtTest.begin(), vtTest.end(), [](int num) {cout << num << " "; }); cout << endl;
	system("pause");
}

结果如下：

90 70 80 60 10 40 50 30 20
80 70 50 60 10 40 20 30
--------------大堆下排序-------------
10 20 30 40 50 60 70 80
--------------实现小堆下排序-------------
80 70 60 50 40 30 20 10