Python的最小堆

Min-Heap 是一个完整的二叉树，其中每个内部节点中的值小于或等于该节点的子节点中的值。
将堆的元素映射到数组是微不足道的：如果节点存储在索引 k 处，则其左子节点存储在索引2k + 1 处，其右子节点存储在索引2k + 2 处。

最小堆示例：

5                      13
         /      \               /       \  
       10        15           16         31 
      /                      /  \        /  \
    30                     41    51    100   41

最小堆是如何表示的?
最小堆是一棵完全二叉树。最小堆通常表示为一个数组。根元素将在Arr[0] 处。对于任何第 i 个节点，即Arr[i] ：

• Arr[(i -1) / 2]返回其父节点。
• Arr[(2 * i) + 1]返回其左子节点。
• Arr[(2 * i) + 2]返回其右子节点。

最小堆上的操作：

1. getMin() ：它返回最小堆的根元素。此操作的时间复杂度为O(1) 。
2. extractMin() ：从 MinHeap 中移除最小元素。此操作的时间复杂度为O(Log n)，因为此操作需要在移除 root 后维护堆属性(通过调用 heapify())。
3. insert() ：插入新键需要O(Log n)时间。我们在树的末尾添加一个新键。如果新键大于它的父键，那么我们不需要做任何事情。否则，我们需要向上遍历以修复违反的堆属性。

下面是最小堆在Python——

Python3

# Python3 implementation of Min Heap
 
import sys
 
class MinHeap:
 
    def __init__(self, maxsize):
        self.maxsize = maxsize
        self.size = 0
        self.Heap = [0]*(self.maxsize + 1)
        self.Heap[0] = -1 * sys.maxsize
        self.FRONT = 1
 
    # Function to return the position of
    # parent for the node currently
    # at pos
    def parent(self, pos):
        return pos//2
 
    # Function to return the position of
    # the left child for the node currently
    # at pos
    def leftChild(self, pos):
        return 2 * pos
 
    # Function to return the position of
    # the right child for the node currently
    # at pos
    def rightChild(self, pos):
        return (2 * pos) + 1
 
    # Function that returns true if the passed
    # node is a leaf node
    def isLeaf(self, pos):
        if pos >= (self.size//2) and pos <= self.size:
            return True
        return False
 
    # Function to swap two nodes of the heap
    def swap(self, fpos, spos):
        self.Heap[fpos], self.Heap[spos] = self.Heap[spos], self.Heap[fpos]
 
    # Function to heapify the node at pos
    def minHeapify(self, pos):
 
        # If the node is a non-leaf node and greater
        # than any of its child
        if not self.isLeaf(pos):
            if (self.Heap[pos] > self.Heap[self.leftChild(pos)] or
               self.Heap[pos] > self.Heap[self.rightChild(pos)]):
 
                # Swap with the left child and heapify
                # the left child
                if self.Heap[self.leftChild(pos)] < self.Heap[self.rightChild(pos)]:
                    self.swap(pos, self.leftChild(pos))
                    self.minHeapify(self.leftChild(pos))
 
                # Swap with the right child and heapify
                # the right child
                else:
                    self.swap(pos, self.rightChild(pos))
                    self.minHeapify(self.rightChild(pos))
 
    # Function to insert a node into the heap
    def insert(self, element):
        if self.size >= self.maxsize :
            return
        self.size+= 1
        self.Heap[self.size] = element
 
        current = self.size
 
        while self.Heap[current] < self.Heap[self.parent(current)]:
            self.swap(current, self.parent(current))
            current = self.parent(current)
 
    # Function to print the contents of the heap
    def Print(self):
        for i in range(1, (self.size//2)+1):
            print(" PARENT : "+ str(self.Heap[i])+" LEFT CHILD : "+
                                str(self.Heap[2 * i])+" RIGHT CHILD : "+
                                str(self.Heap[2 * i + 1]))
 
    # Function to build the min heap using
    # the minHeapify function
    def minHeap(self):
 
        for pos in range(self.size//2, 0, -1):
            self.minHeapify(pos)
 
    # Function to remove and return the minimum
    # element from the heap
    def remove(self):
 
        popped = self.Heap[self.FRONT]
        self.Heap[self.FRONT] = self.Heap[self.size]
        self.size-= 1
        self.minHeapify(self.FRONT)
        return popped
 
# Driver Code
if __name__ == "__main__":
     
    print('The minHeap is ')
    minHeap = MinHeap(15)
    minHeap.insert(5)
    minHeap.insert(3)
    minHeap.insert(17)
    minHeap.insert(10)
    minHeap.insert(84)
    minHeap.insert(19)
    minHeap.insert(6)
    minHeap.insert(22)
    minHeap.insert(9)
    minHeap.minHeap()
 
    minHeap.Print()
    print("The Min val is " + str(minHeap.remove()))

输出 ：

The Min Heap is 
 PARENT : 3 LEFT CHILD : 5 RIGHT CHILD :6
 PARENT : 5 LEFT CHILD : 9 RIGHT CHILD :84
 PARENT : 6 LEFT CHILD : 19 RIGHT CHILD :17
 PARENT : 9 LEFT CHILD : 22 RIGHT CHILD :10
The Min val is 3

使用库函数：

我们使用 heapq 类在Python实现堆。默认情况下，Min Heap 由此类实现。

蟒蛇3

# Python3 program to demonstrate working of heapq
 
from heapq import heapify, heappush, heappop
 
# Creating empty heap
heap = []
heapify(heap)
 
# Adding items to the heap using heappush function
heappush(heap, 10)
heappush(heap, 30)
heappush(heap, 20)
heappush(heap, 400)
 
# printing the value of minimum element
print("Head value of heap : "+str(heap[0]))
 
# printing the elements of the heap
print("The heap elements : ")
for i in heap:
    print(i, end = ' ')
print("\n")
 
element = heappop(heap)
 
# printing the elements of the heap
print("The heap elements : ")
for i in heap:
    print(i, end = ' ')

输出 ：

Head value of heap : 10
The heap elements : 
10 30 20 400 

The heap elements : 
20 30 400

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