Algorithms Problem Solving: Maximum 69 Number

This post is part of the Algorithms Problem Solving series.

Problem description

This is the Maximum 69 Number problem. The description looks like this:

Given a positive integer num consisting only of digits 6 and 9.

Return the maximum number you can get by changing at most one digit (6 becomes 9, and 9 becomes 6).

Examples

Input: num = 9669
Output: 9969

Input: num = 9996
Output: 9999

Input: num = 9999
Output: 9999

Solution

The idea is to find the first 6 number index in the whole number. And then replace it with 9. That way you get the maximum number.

def maximum_69_number(num):
    six_index = 0
    str_num = str(num)

    for index in range(len(str_num)):
        if str_num[index] == '6':
            six_index = index
            break

    return int(
        str_num[:six_index] +
        '9' +
        str_num[six_index + 1:]
    )

You can also use the replace string method to replace the first 6 number with 9.

'6996'.replace('6', '9', 1)
'9996'

'9696'.replace('6', '9', 1)
'9996'

Resources

• Learning Python: From Zero to Hero
• Algorithms Problem Solving Series
• Stack Data Structure
• Queue Data Structure
• Linked List
• Tree Data Structure