优先队列

　　优先队列是由堆实现的。

#include <iostream>
#include <fstream>
#include <string>

template<typename T>
void swap(T &x, T &y) {
    T temp = x;
    x = y;
    y = temp;
}

const int inf = (1LL << 31) - 1;
const int maxn = 10005;
int arr[maxn];
int size; 


void max_heapinf(int *arr, int i, int length) // 维护堆的性质
{
    int l = 2 * i, r = l + 1;
    int largest = i;
    if (l <= length && arr[l] > arr[largest])
        largest = l;
    if (r <= length && arr[r] > arr[largest])
        largest = r;
    if (largest != i)
    {
        swap(arr[largest], arr[i]);
        max_heapinf(arr, largest, length);
    }
}

// 优先队列部分
int heap_maximum(int *arr) // 返回队列中具有最大关键字的元素
{
    return arr[1];
}

int heap_extract_max(int *arr) // 去掉并返回队列中最大关键字的元素
{
    if (size < 1)
    {
        std::cout << "heap underflow" << std::endl;
        return 0;
    }
    int ret = arr[1];
    arr[1] = arr[size--];
    max_heapinf(arr, 1, size);
    return ret;
}

void heap_increase_key(int *arr, int i, int key) //要求 key >= arr[i]
{
    if (key < arr[i])
    {
        std::cout << "new key is smaller than current key" << std::endl;
        return;
    }
    arr[i] = key;
    while (i > 1 && arr[i] > arr[i/2])
    {
        swap(arr[i], arr[i / 2]);
        i /= 2;
    }
}

void heap_insert(int *arr, int key)
{
    arr[++size] = -inf;
    heap_increase_key(arr, size, key);
}

void print_arr(const int *arr,int l, int r)
{
    for (int i = l; i <= r; ++i)
        std::cout << arr[i] << " ";
    std::cout << std::endl;
}
int main()
{
    for (int i = 1; i <= 10; ++i)
        heap_insert(arr, i);
    for (int i = 1; i <= 10; ++i)
        heap_insert(arr, -i);
    std::cout << heap_extract_max(arr) << std::endl;
    heap_increase_key(arr, 3, 100);
    std::cout << heap_extract_max(arr) << std::endl;
    return 0;
}