Array based MinHeap

Question

I am trying to implement a function that turns a list into a MinHeap. I cannot see where I am going wrong.

def min_heapify(nums):
    n = len(nums) - 1
    i = n // 2  # last parent

    while i >= 1:
        k = i
        v = nums[k]
        min_heap = False
        while not min_heap and 2 * k <= n:
            j = 2 * k
            if j + 1 <= n:
                if nums[j + 1] < nums[j]:
                    j += 1
            if nums[k] < nums[j]:
                min_heap = True
            else:
                nums[k] = nums[j]
                k = j
        nums[k] = v
        i -= 1

Example Input:

a_list = [0, 5, 2, 3, 4]
min_heapify(a_list)

Output (the 4 is incorrect):

print("Min Heapify", a_list)  # Min Heapify [0, 2, 5, 3, 4]

Answer 1

I think you should be swaping num[j] and num[k], where you are just assigning nums[k] = nums[j]:

def min_heapify(nums):
    n = len(nums) - 1
    i = n // 2  # last parent

    while i >= 1:
        k = i
        v = nums[k]
        min_heap = False
        while not min_heap and 2 * k <= n:
            j = 2 * k
            if j + 1 <= n:
                if nums[j + 1] < nums[j]:
                    j += 1
            if nums[k] < nums[j]:
                min_heap = True
            else:
                # HERE: Need to swap num[j] and num[k]
                [nums[j], nums[k]] = [nums[k], nums[j]]
                k = j
        nums[k] = v
        i -= 1

a_list = [0, 11, 5, 2, 3, 4, 7, 17, 6, 1]
min_heapify(a_list)
print(a_list)

# prints: [0, 1, 3, 2, 5, 4, 7, 17, 6, 11]