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TK

Posted on • Originally published at leandrotk.github.io

# 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'
``````