STL heap usage

简介

　　heap有查找时间复杂度O(1),查找、插入、删除时间复杂度为O(logN)的特性，STL中heap相关的操作如下：

　　make_heap()

　　push_heap()

　　pop_heap()

　　sort_heap()

　　reverse(）

本次着重介绍make_heap() ,根据其创出的堆有大小堆之分。 其函数原型如下：

template <class RandomAccessIterator>
  void make_heap (RandomAccessIterator first, RandomAccessIterator last);

template <class RandomAccessIterator, class Compare>
  void make_heap (RandomAccessIterator first, RandomAccessIterator last,
                  Compare comp );

　　函数解释如下：

　　Make heap from range

　　Rearranges the elements in the range [first,last) in such a way that they form a heap.

　　A heap is a way to organize the elements of a range that allows for fast retrieval of the element with the highest value at any moment (with pop_heap), even repeatedly, while allowing for fast insertion of new elements (with 　　　　　　　　push_heap).

　　The element with the highest value is always pointed by first. The order of the other elements depends on the particular implementation, but it is consistent throughout all heap-related functions of this header.

　　The elements are compared using operator< (for the first version), or comp (for the second): The element with the highest value is an element for which this would return false when compared to every other element in the range.

　　The standard container adaptorpriority_queue callsmake_heap,push_heap andpop_heap automatically to maintain heap properties for a container.

　　Parameters

　　first, last

Random-access iterators to the initial and final positions of the sequence to be transformed into a heap. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
RandomAccessIterator shall point to a type for whichswap is properly defined and which is both move-constructible and move-assignable.

　　comp

Binary function that accepts two elements in the range as arguments, and returns a value convertible to bool. The value returned indicates whether the element passed as first argument is considered to be less than the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

基本上可以概括为默认的参数原型创建最大堆，带有comp仿函数参数的原型可以根据comp的选取创建所需的堆，根据后面的实例可得，默认的比较符是小于号，创建最大堆； 若comp的比较操作为“ >” 则创建最小堆。

实例

　　1.创建最大堆（默认函数）

// range heap example
#include <iostream>     // std::cout
#include <algorithm>    // std::make_heap, std::pop_heap, std::push_heap, std::sort_heap
#include <vector>       // std::vector

int main () {
  int myints[] = {10,20,30,5,15};
  std::vector<int> v(myints,myints+5);

  std::make_heap (v.begin(),v.end());
  std::cout << "initial max heap   : " << v.front() << ‘\n‘;

  std::pop_heap (v.begin(),v.end()); v.pop_back();
  std::cout << "max heap after pop : " << v.front() << ‘\n‘;

  v.push_back(99); std::push_heap (v.begin(),v.end());
  std::cout << "max heap after push: " << v.front() << ‘\n‘;

  std::sort_heap (v.begin(),v.end());

  std::cout << "final sorted range :";
  for (unsigned i=0; i<v.size(); i++)
    std::cout << ‘ ‘ << v[i];

  std::cout << ‘\n‘;

  return 0;
}

输出：

initial max heap   : 30
max heap after pop : 20
max heap after push: 99
final sorted range : 5 10 15 20 99

可得最大堆。

　　2.最小堆创建（使用custom（2)函数）

#include <iostream>
#include <vector>
#include <algorithm>

struct doc {
    double rank;
    explicit doc(double r) : rank(r) {}
};

struct doc_rank_greater_than {
    bool operator()(doc const& a, doc const& b) const {
        return a.rank > b.rank;
    }
};

int main() {
    std::vector<doc> docvec;
    docvec.push_back( doc(4) );
    docvec.push_back( doc(3) );
    docvec.push_back( doc(2) );
    docvec.push_back( doc(1) );
    std::make_heap(docvec.begin(),docvec.end(),doc_rank_greater_than());
    std::cout << docvec.front().rank << ‘\n‘;
}

输出：

1

可得最小堆。

总结　　自定义函数可以根据容器中的对象创建相应的最大最小堆。