Maximum 69 Number

Problem Statement

In this task, you are presented with a non-negative integer called num, which is formed only by the digits 6 and 9. The goal is to derive the maximum possible number from num by altering at most one digit in num. This alteration involves switching a '6' to '9', or vice versa. If no such switch results in a higher number, the number should remain unchanged. The challenge lies not just in switching the digits, but determining where the switch should occur (if at all) to guarantee that num becomes as large as possible.

Examples

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 <= 104
• num consists of only 6 and 9 digits.

Approach and Intuition

1. The primary goal is to transform the integer num into its maximum form by changing a digit from '6' to '9'. Conversely, transforming '9' to '6' generally reduces the value, which is counterproductive under the task's requirements.

2. You can approach this problem by:

   • Converting the number to a string or array format for easier manipulation of individual digits.

   • Iteratively examining each digit from the leftmost to the rightmost:

     • If the digit is '6', change it to '9'. Convert the modified string or array back to a number, then compare it with the original number and all previously generated numbers to determine which is largest.
     • If the digit is '9', simply move on to the next digit because switching it to '6' would reduce the number's value.
     • Record the maximum value found during these transformations.

   • Consider the highest value obtained from these comparisons as the final result.

3. Edge Cases:

   • If the number consists solely of the digit '9', any transformation would only maintain or lower its value. In such scenarios, the original number is already the largest possible.
   • If the number consists solely of the digit '6', converting the first '6' to '9' will yield the largest possible value for the number.

In this manner, the algorithm ensures that the number is increased maximally with minimal transformations, adhering strictly to the constraints and the requirement of changing at most one digit.

Solutions

class Solution {
public:
    int highest69Number(int number) {
        int originalNumber = number;
        int positionOfSix = -1;
        int currentPlace = 0;
    
        while (originalNumber > 0) {
            if (originalNumber % 10 == 6)
                positionOfSix = currentPlace;
    
            originalNumber /= 10;
            currentPlace++;
        }
    
        return positionOfSix == -1 ? number : number + 3 * (int)pow(10, positionOfSix);
    }
};