STL源码：heap

港控/mmm° 2022-08-07 08:41 231阅读 0赞

heap不归属STL容器组件，它是priority queue的幕后英雄。而**heap的底层可以用array或vector来实现**。**STL实现了大根堆，使用的是vector作为底部容器**。

**二叉堆其实是一个以vector表现的完全二叉树**。

**为什么用二叉大根堆作为优先队列的底层机制？**

答：优先队列允许用户以任何次序将任何元素插入容器，但取出时要从优先权最大的元素开始取。二叉大根堆具有这样的特性，因此作为优先队列的底层机制。

关于二叉堆实现优先队列的可行性见《[二叉堆的插入删除等操作C++实现][C]》

**能不能用list作为优先队列底层机制？**

答：list作为优先队列底层机制可以实现元素插入的常数时间，但是要找到list的最值，却要对list做线性扫描。我们可以先对元素排序，这样，找到最值以及元素删除为常数时间，但是元素的插入又是线性时间。

**能不能用二叉搜索树作为优先队列底层机制？**

答：用二叉搜索树元素插入和最值取得都可以在O(logn)内完成，但是这样小题大做：第一，**二叉搜索树的输入需要足够的随机性**；第二，二叉搜索树的实现不容易。

# heap源码 #

STL实现的是大根堆，底层容器为vector，使用迭代器。

之前使用vector作为底层容器，使用下标，自己实现过小根堆的操作[堆的插入删除操作][C]，以及[堆排序][Link 1]

//stl_heap.h
    #ifndef __SGI_STL_INTERNAL_HEAP_H
    #define __SGI_STL_INTERNAL_HEAP_H
    
    __STL_BEGIN_NAMESPACE
    
    #if defined(__sgi) && !defined(__GNUC__) && (_MIPS_SIM != _MIPS_SIM_ABI32)
    #pragma set woff 1209
    #endif
    
    //
    //push_heap()操作前要保证新添加的元素已经加入到容器末尾!!!
    ///
    template <class RandomAccessIterator, class Distance, class T>
    void __push_heap(RandomAccessIterator first, Distance holeIndex,
                     Distance topIndex, T value)
    {
      // 首先找出待处理元素的父结点
      Distance parent = (holeIndex - 1) / 2;
    
      // 判断当前待处理结点是否优先级高于其父结点, 如果是则将其父结点向下移动
      // 设置当前结点为父结点位置, 继续, 直到优先级小于父结点或者已经到达heap顶端
      while (holeIndex > topIndex && *(first + parent) < value) {
    	  //当尚未到达顶端且父节点小于新值
        *(first + holeIndex) = *(first + parent); //令洞值为父值
        holeIndex = parent;  //调整洞号，向上提升至父节点
        parent = (holeIndex - 1) / 2;  //新洞的父节点
      }
      // 将找到的合适的位置设置成正确值
      *(first + holeIndex) = value;
    }
    
    template <class RandomAccessIterator, class Distance, class T>
    inline void __push_heap_aux(RandomAccessIterator first,
                                RandomAccessIterator last, Distance*, T*)
    {
      // 因为first所指的那个元素不是heap的组成元素, 所以计算距离要减去1
      __push_heap(first, Distance((last - first) - 1), Distance(0),
                  T(*(last - 1)));
    }
    
    // 调用此函数前要先把待处理元素追加到容器末尾
    template <class RandomAccessIterator>
    inline void push_heap(RandomAccessIterator first, RandomAccessIterator last)
    {
      __push_heap_aux(first, last, distance_type(first), value_type(first));
    }
    
    template <class RandomAccessIterator, class Distance, class T, class Compare>
    void __push_heap(RandomAccessIterator first, Distance holeIndex,
                     Distance topIndex, T value, Compare comp)
    {
      Distance parent = (holeIndex - 1) / 2;
      while (holeIndex > topIndex && comp(*(first + parent), value)) {
        *(first + holeIndex) = *(first + parent);
        holeIndex = parent;
        parent = (holeIndex - 1) / 2;
      }
      *(first + holeIndex) = value;
    }
    
    template <class RandomAccessIterator, class Compare, class Distance, class T>
    inline void __push_heap_aux(RandomAccessIterator first,
                                RandomAccessIterator last, Compare comp,
                                Distance*, T*)
    {
      __push_heap(first, Distance((last - first) - 1), Distance(0),
                  T(*(last - 1)), comp);
    }
    
    // 这个除了用户自己指定优先级决策判别式外和默认的无区别
    template <class RandomAccessIterator, class Compare>
    inline void push_heap(RandomAccessIterator first, RandomAccessIterator last,
                          Compare comp)
    {
      __push_heap_aux(first, last, comp, distance_type(first), value_type(first));
    }
    
    /
    // 注意: pop_heap()操作, 执行完操作后要自己将容器尾元素弹出
    //
    // 这里以默认的heap优先级决策来说
    // STL采用的是先将待pop的元素复制到heap尾部, 然后将整个heap向上调整
    // 这样就会将最后空出一个hole, 将原来最后的元素在这里进行push()操作
    // 这就是两个shift_up的过程
    // 个人感觉使用使用shift_down的算法更高效, 虽然时间复杂度一样, 但是shift_down
    // 进行操作的元素会更少,
    // 之所以用shift_up这可能也是STL设计理念的问题吧, 能复用就不写新的^_^
    
    template <class RandomAccessIterator, class Distance, class T>
    void __adjust_heap(RandomAccessIterator first, Distance holeIndex,
                       Distance len, T value)
    {
      Distance topIndex = holeIndex;
      Distance secondChild = 2 * holeIndex + 2;     // 弹出元素的有子孩
    
      // 调整heap元素位置
      while (secondChild < len) {
        // 选择两个子孩中较大的进行操作, 使用secondChild表示其偏移
        if (*(first + secondChild) < *(first + (secondChild - 1)))
          secondChild--;
    
        // 将较大元素向上填充, 并将整体偏移向下调整, 继续调整
        *(first + holeIndex) = *(first + secondChild);
        holeIndex = secondChild;
        secondChild = 2 * (secondChild + 1);
      }
    
      if (secondChild == len) {
        *(first + holeIndex) = *(first + (secondChild - 1));
        holeIndex = secondChild - 1;
      }
    
      // 这里就是shift_up过程了, 将最初的heap末尾元素向上调整
      // 侯捷老师对这里的理解有误, :-), 人非圣贤, 孰能无过, ^_^
      __push_heap(first, holeIndex, topIndex, value);
    }
    
    template <class RandomAccessIterator, class T, class Distance>
    inline void __pop_heap(RandomAccessIterator first, RandomAccessIterator last,
                           RandomAccessIterator result, T value, Distance*)
    {
      // 将弹出的元素调整到heap末尾, 这个元素需要用户手动弹出
      *result = *first;
    
      // 去掉末尾哪个弹出的元素, 调整heap
      __adjust_heap(first, Distance(0), Distance(last - first), value);
    }
    
    template <class RandomAccessIterator, class T>
    inline void __pop_heap_aux(RandomAccessIterator first,
                               RandomAccessIterator last, T*)
    {
      __pop_heap(first, last - 1, last - 1, T(*(last - 1)), distance_type(first));
    }
    
    template <class RandomAccessIterator>
    inline void pop_heap(RandomAccessIterator first, RandomAccessIterator last)
    {
      __pop_heap_aux(first, last, value_type(first));
    }
    
    template <class RandomAccessIterator, class Distance, class T, class Compare>
    void __adjust_heap(RandomAccessIterator first, Distance holeIndex,
                       Distance len, T value, Compare comp)
    {
      Distance topIndex = holeIndex;
      Distance secondChild = 2 * holeIndex + 2; //洞节点的右孩子节点
      while (secondChild < len) {
    	  //比较洞节点左右两孩子，然后以secondChild代表大的子节点
        if (comp(*(first + secondChild), *(first + (secondChild - 1))))
          secondChild--;
    	//下滤：令较大的孩子值为洞值，令洞号下移到较大的孩子节点
        *(first + holeIndex) = *(first + secondChild); 
        holeIndex = secondChild; 
        secondChild = 2 * (secondChild + 1); //找到新洞节点的右孩子节点
      }
      if (secondChild == len) { //没有右孩子，只有左孩子
    	  //下滤：令左孩子为洞值，再令洞下移到左孩子节点
        *(first + holeIndex) = *(first + (secondChild - 1));
        holeIndex = secondChild - 1;
      }
       //将欲调整值插入目前的洞内
      __push_heap(first, holeIndex, topIndex, value, comp);
    }
    
    template <class RandomAccessIterator, class T, class Compare, class Distance>
    inline void __pop_heap(RandomAccessIterator first, RandomAccessIterator last,
                           RandomAccessIterator result, T value, Compare comp,
                           Distance*)
    {
      *result = *first;
      __adjust_heap(first, Distance(0), Distance(last - first), value, comp);
    }
    
    template <class RandomAccessIterator, class T, class Compare>
    inline void __pop_heap_aux(RandomAccessIterator first,
                               RandomAccessIterator last, T*, Compare comp)
    {
      __pop_heap(first, last - 1, last - 1, T(*(last - 1)), comp,
                 distance_type(first));
    }
    
    template <class RandomAccessIterator, class Compare>
    inline void pop_heap(RandomAccessIterator first, RandomAccessIterator last,
                         Compare comp)
    {
        __pop_heap_aux(first, last, value_type(first), comp);
    }
    
    // 建立堆的过程就是一系列插入，即下滤过程
    template <class RandomAccessIterator, class T, class Distance>
    void __make_heap(RandomAccessIterator first, RandomAccessIterator last, T*,
                     Distance*)
    {
      if (last - first < 2) return;  //堆仅含0或1个元素，不必重新排序
      Distance len = last - first;
      Distance parent = (len - 2)/2;
    
      while (true) {
        __adjust_heap(first, parent, len, T(*(first + parent)));
        if (parent == 0) return; //走完了根节点，结束
        parent--; 
      }
    }
    
    template <class RandomAccessIterator>
    inline void make_heap(RandomAccessIterator first, RandomAccessIterator last)
    {
      __make_heap(first, last, value_type(first), distance_type(first));
    }
    
    template <class RandomAccessIterator, class Compare, class T, class Distance>
    void __make_heap(RandomAccessIterator first, RandomAccessIterator last,
                     Compare comp, T*, Distance*)
    {
      if (last - first < 2) return;
      Distance len = last - first;
      Distance parent = (len - 2)/2;
    
      while (true) {
        __adjust_heap(first, parent, len, T(*(first + parent)), comp);
        if (parent == 0) return;
        parent--;
      }
    }
    
    template <class RandomAccessIterator, class Compare>
    inline void make_heap(RandomAccessIterator first, RandomAccessIterator last,
                          Compare comp)
    {
      __make_heap(first, last, comp, value_type(first), distance_type(first));
    }
    
    // 堆排序，保证heap有序，每次将堆的最值放在vector的末尾
    template <class RandomAccessIterator>
    void sort_heap(RandomAccessIterator first, RandomAccessIterator last)
    {
      while (last - first > 1) pop_heap(first, last--);
    }
    
    template <class RandomAccessIterator, class Compare>
    void sort_heap(RandomAccessIterator first, RandomAccessIterator last,
                   Compare comp)
    {
      while (last - first > 1) pop_heap(first, last--, comp);
    }
    
    #if defined(__sgi) && !defined(__GNUC__) && (_MIPS_SIM != _MIPS_SIM_ABI32)
    #pragma reset woff 1209
    #endif
    
    __STL_END_NAMESPACE
    
    #endif /* __SGI_STL_INTERNAL_HEAP_H */

[C]: http://blog.csdn.net/u013074465/article/details/42028473
[Link 1]: http://blog.csdn.net/u013074465/article/details/42043697