heaps in python code example

Example 1: heapq python how to use comparator

class Solution:
    def mergeKLists(self, lists: List[ListNode]) -> ListNode:
        
        setattr(ListNode, "__lt__", lambda self, other: self.val <= other.val)
            
        pq = []
        for l in lists:
            if l:
                heapq.heappush(pq,  l)
        
        out = ListNode(None)
        head = out
        while pq:
            l = heapq.heappop(pq)
            head.next = l
            head = head.next
            if l and l.next:
                heapq.heappush( pq, l.next)
            
        return out.next

Example 2: python heapq

>>> import heapq
>>> heap = []
>>> heapq.heappush(heap, (5, 'write code'))
>>> heapq.heappush(heap, (7, 'release product'))
>>> heapq.heappush(heap, (1, 'write spec'))
>>> heapq.heappush(heap, (3, 'create tests'))
>>> heapq.heappop(heap)#pops smallest
(1, 'write spec')
>>> heapq.nlargest(2,heap)#displays n largest values without popping
[(7, 'release product'),(5, 'write code')]
>>> heapq.nsmallest(2,heap)#displays n smallest values without popping
[(3, 'create tests'),(5, 'write code')]
>>> heap = [1, 3, 5, 7, 9, 2, 4, 6, 8, 0]
>>> heapq.heapify(heap)#converts a list to heap
>>> heap
[0, 1, 2, 6, 3, 5, 4, 7, 8, 9]
>>> def heapsort(iterable):
...     h = []
...     for value in iterable:
...         heappush(h, value)
...     return [heappop(h) for i in range(len(h))]
...
>>> heapsort([1, 3, 5, 7, 9, 2, 4, 6, 8, 0])
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Example 3: python code for heap using heapify

#Implementing Heap Using Heapify Method in Python 3
#MaxHeapify,MinHeapify,Ascending_Heapsort,Descending_Heapsort
class heap:
    
    def maxheapify(self,array):
        n=len(array)
        for i in range(n//2-1,-1,-1):
            self._maxheapify(array,n,i)
            
            
    def _maxheapify(self,array,n,i):
        l=2*i+1
        r=2*i+2
        if l<n and array[l]>array[i]:
            largest=l
        else:
            largest=i
        if r<n and array[r]>array[largest]:
            largest=r
        if (largest!=i):
            array[largest],array[i]=array[i],array[largest]
            self._maxheapify(array,n,largest)
            
            
    def minheapify(self,array):
        n = len(array)
        for i in range(n//2-1,-1,-1):
            self._minheapify(array,n,i)
            
            
    def _minheapify(self,array,n,i):
        l=2*i+1
        r=2*i+2
        if l<n and array[l]<array[i]:
            smallest = l
        else:
            smallest = i
        if r < n and array[r]<array[smallest]:
            smallest = r
        if (smallest != i):
            array[smallest], array[i] = array[i], array[smallest]
            self._minheapify(array, n, smallest)
            
            
    def descending_heapsort(self,array):
        n = len(array)
        for i in range(n // 2 - 1, -1, -1):
            self._minheapify(array, n, i)
        for i in range(n - 1, 0, -1):
            array[0], array[i] = array[i], array[0]
            self._minheapify(array, i, 0)


    def ascending_heapsort(self,array):
        n=len(array)
        for i in range(n//2-1,-1,-1):
            self._maxheapify(array,n,i)
        for i in range(n-1,0,-1):
            array[0],array[i]=array[i],array[0]
            self._maxheapify(array,i,0)

b=[550,4520,3,2340,12]
a=heap()

a.maxheapify(b)
print('Max Heapify -->',b)

a.minheapify(b)
print('Min Heapify -->',b)

a.ascending_heapsort(b)
print('Ascending Heap Sort -->',b)

a.descending_heapsort(b)
print('Descending Heap Sort -->',b)

Example 4: Heap in python

Heap Implementation at this link:

https://github.com/shreyasvedpathak/Data-Structure-Python/tree/master/Hashing