最大堆

做online judge的时候用到了最大堆，自己实现了一个。主要参考算法导论中的设计思路。

#include <iostream>
using namespace std;

const int MAX_LEN = 100;

class MaxHeap
{
private:
    int heap[MAX_LEN];
    int len;
    inline int parent(int i){ return i / 2; };
    inline int left(int i){ return i * 2; };
    inline int right(int i){ return i * 2 + 1; };
    void heapify(int i);
    void build(int *arr, int n);
public:
    MaxHeap();
    MaxHeap(int *arr, int n);
    inline int length(){ return len; };
    inline int max(){ return heap[1]; };
    void print();
    int extractMax();
    void increaseKey(int i, int key);
    void insert(int key);
};

MaxHeap::MaxHeap(){
    len = 0;
    memset(heap, 0, MAX_LEN);
}

MaxHeap::MaxHeap(int *arr, int n){
    len = n;
    memset(heap, 0, MAX_LEN);
    build(arr, n);
}

void MaxHeap::heapify(int i){
    int l = left(i);
    int r = right(i);
    int largest = i;
    if (l <= len&&heap[l] > heap[largest])
        largest = l;
    if (r <= len&&heap[r] > heap[largest])
        largest = r;
    if (largest != i){
        swap(heap[i], heap[largest]);
        heapify(largest);
    }
}

void MaxHeap::build(int *arr, int n){
    for (int i = 0; i < n; i++)
        heap[i + 1] = arr[i];
    for (int i = len / 2; i >= 1; i--)
        heapify(i);
}

int MaxHeap::extractMax(){
    if (len < 1){
        cout << "Error. Returning -1." << endl;
        return -1;
    }
    int max = heap[1];
    heap[1] = heap[len];
    len--;
    heapify(1);
    return max;
}

void MaxHeap::print(){
    for (int i = 1; i <= len; i++)
        cout << heap[i] << endl;
}

void MaxHeap::increaseKey(int i, int key){
    if (key < heap[i]){
        cout << "New key is smaller than current key." << endl;
        return;
    }
    heap[i] = key;
    while (i > 1 && heap[parent(i)] < heap[i]){
        swap(heap[parent(i)], heap[i]);
        i = parent(i);
    }
}

void MaxHeap::insert(int key){
    len++;
    heap[len] = INT_MIN;
    increaseKey(len, key);
}

int main()
{
    int num[10] = { 4, 1, 6, 2, 9, 7, 3, 8, 5, 0 };
    MaxHeap max_heap(num, 10);
    max_heap.print();
    return 0;
}