The create_max function takes an input number num as a string and an integer k as parameters, returning the maximum number that can be formed by removing digits from num and keeping at most k digits. The function uses a stack to iteratively select the largest digits from num until a maximum number of k digits have been chosen.

Cell 5

def create_max(num, k):
    digits = list(num)
    drop = len(digits) - k
    stack = []
    for digit in digits:
        while drop and stack and stack[-1] < digit:
            stack.pop()
            drop -= 1
        stack.append(digit)
    return ''.join(stack[:k])

num = '912583'
assert create_max(num, 1) == '9'
assert create_max(num, 2) == '98'
assert create_max(num, 3) == '983'
assert create_max(num, 4) == '9583'
assert create_max(num, 5) == '92583'
print('All passed!')

def create_max(num: str, k: int) -> str:
    """
    Returns the k largest digits from the input number, in descending order.
    
    Args:
        num (str): The input number as a string.
        k (int): The number of largest digits to return.
    
    Returns:
        str: The k largest digits from the input number, in descending order.
    """
    # Convert the input number to a list of digits for easier manipulation
    digits = list(num)
    
    # Calculate the number of digits to drop from the end of the list
    drop = len(digits) - k
    
    # Initialize an empty stack to store the digits
    stack = []
    
    # Iterate over each digit in the list
    for digit in digits:
        # While there are digits to drop from the stack and the top of the stack is less than the current digit
        while drop and stack and stack[-1] < digit:
            # Remove the top digit from the stack
            stack.pop()
            # Decrement the number of digits to drop
            drop -= 1
        # Push the current digit onto the stack
        stack.append(digit)
    
    # Return the k largest digits from the stack as a string
    return ''.join(stack[:k])

# Test cases
num = '912583'
assert create_max(num, 1) == '9'
assert create_max(num, 2) == '98'
assert create_max(num, 3) == '983'
assert create_max(num, 4) == '9583'
assert create_max(num, 5) == '92583'

print('All passed!')

Code Breakdown

Function: create_max

Parameters

• num: the input number as a string
• k: the number of digits to be kept

Description

The function create_max returns the maximum number that can be formed by removing digits from the input number num and keeping at most k digits.

Implementation

1. Convert the input number num to a list of digits.
2. Initialize a stack to store the digits to be kept.
3. Iterate over each digit in the list of digits.
4. While the stack is not empty and the top of the stack is less than the current digit, and we still have digits to remove (drop is greater than 0):
   • Remove the top of the stack.
   • Decrement drop by 1.
5. Append the current digit to the stack.
6. Repeat steps 4-5 until all digits have been processed.
7. Return the maximum number that can be formed by joining the top k digits from the stack.

Example Usage

The code includes several assertions to test the function with different inputs and expected outputs. The function is then called with the input number '912583' and k values from 1 to 5, and the results are printed.