# Algorithms Problem Solving: Maximum 69 Number

This post is part of the [Algorithms Problem Solving](https://leandrotk.github.io/tk/series/algorithms-problem-solving/index.html) series.

## Problem description

This is the [Maximum 69 Number](https://leetcode.com/problems/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

```python
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.

```python
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`.

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

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

## Resources

- [Learning Python: From Zero to Hero](https://leandrotk.github.io/tk/2017/09/learning-python-from-zero-to-hero/index.html)
- [Algorithms Problem Solving Series](https://leandrotk.github.io/tk/series/algorithms-problem-solving/index.html)
- [Stack Data Structure](https://leandrotk.github.io/tk/2020/01/stack-data-structure/index.html)
- [Queue Data Structure](https://leandrotk.github.io/tk/2020/01/queue-data-structure/index.html)
- [Linked List](https://leandrotk.github.io/tk/2020/02/linked-list/index.html)
- [Tree Data Structure](https://leandrotk.github.io/tk/2020/02/tree-data-structure/index.html)