Problem

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).

Example 1:

Input: num = 9669

Output: 9969

Explanation:

Changing the first digit results in 6669. Changing the second digit results in 9969. Changing the third digit results in 9699. Changing the fourth digit results in 9666.  The maximum number is 9969.

Example 2:

Input: num = 9996

Output: 9999

Explanation: Changing the last digit 6 to 9 results in the maximum number.

Example 3:

Input: num = 9999

Output: 9999

Explanation: It is better not to apply any change.

Constraints:

1 <= num <= 10^4 num’s digits are 6 or 9.

Pre analysis

Simple iterate over the number and find & replace first occurence of 9.

Post analysis

Recalling, String.prototype replace function , it replaces only 1 occurence oin the string. That would give 1 liner, although no change in space/time complexity

Another solution

const maximum69Number  = num => num.toString().replace("6", "9")


const maximum69Number  = num => +num.toString().replace(/6/, '9');