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using System;
using System.Collections.Generic;
namespace CollectionsExt
{
// 最小堆，堆顶是最小值。
// 如果需要维护一个优先队列，用堆结构，调堆的效率是O(logn)快于遍历的O(n)
public class MinHeap<T>
{
private const int InitialCapacity = 4;
private T[] _arr;
private int _lastItemIndex;
private IComparer<T> _comparer;
public MinHeap()
: this(InitialCapacity, Comparer<T>.Default)
{
}
public MinHeap(int capacity)
: this(capacity, Comparer<T>.Default)
{
}
public MinHeap(Comparison<T> comparison)
: this(InitialCapacity, Comparer<T>.Create(comparison))
{
}
public MinHeap(IComparer<T> comparer)
: this(InitialCapacity, comparer)
{
}
public MinHeap(int capacity, IComparer<T> comparer)
{
_arr = new T[capacity];
_lastItemIndex = -1;
_comparer = comparer;
}
public int Count
{
get
{
return _lastItemIndex + 1;
}
}
public void Add(T item)
{
if (_lastItemIndex == _arr.Length - 1)
{
Resize();
}
_lastItemIndex++;
_arr[_lastItemIndex] = item;
MinHeapifyUp(_lastItemIndex);
}
public T Remove()
{
if (_lastItemIndex == -1)
{
throw new InvalidOperationException("The heap is empty");
}
T removedItem = _arr[0];
_arr[0] = _arr[_lastItemIndex];
_lastItemIndex--;
MinHeapifyDown(0);
return removedItem;
}
public T Peek()
{
if (_lastItemIndex == -1)
{
throw new InvalidOperationException("The heap is empty");
}
return _arr[0];
}
public void Clear()
{
_lastItemIndex = -1;
}
// 调堆,O(logn)
private void MinHeapifyUp(int index)
{
if (index == 0)
{
return;
}
int childIndex = index;
int parentIndex = (index - 1) / 2;
// 堆顶是最小值
if (_comparer.Compare(_arr[childIndex], _arr[parentIndex]) < 0)
{
// swap the parent and the child
T temp = _arr[childIndex];
_arr[childIndex] = _arr[parentIndex];
_arr[parentIndex] = temp;
// 递归
MinHeapifyUp(parentIndex);
}
}
// 调堆,O(logn)
private void MinHeapifyDown(int index)
{
int leftChildIndex = index * 2 + 1;
int rightChildIndex = index * 2 + 2;
int smallestItemIndex = index; // The index of the parent
if (leftChildIndex <= _lastItemIndex &&
_comparer.Compare(_arr[leftChildIndex], _arr[smallestItemIndex]) < 0)
{
smallestItemIndex = leftChildIndex;
}
if (rightChildIndex <= _lastItemIndex &&
_comparer.Compare(_arr[rightChildIndex], _arr[smallestItemIndex]) < 0)
{
smallestItemIndex = rightChildIndex;
}
if (smallestItemIndex != index)
{
// swap the parent with the smallest of the child items
T temp = _arr[index];
_arr[index] = _arr[smallestItemIndex];
_arr[smallestItemIndex] = temp;
MinHeapifyDown(smallestItemIndex);
}
}
private void Resize()
{
T[] newArr = new T[_arr.Length * 2];
for (int i = 0; i < _arr.Length; i++)
{
newArr[i] = _arr[i];
}
_arr = newArr;
}
}
}
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