kotlin coding test course, sorting digits in descending order

Problem Description

Reconstruct the number by sorting the digits of a given integer N in descending order. Return the number after sorting the digits. The integer N is between 0 and 2³¹-1.

For example:

• Input: 118372 → Output: 873211
• Input: 2143 → Output: 4321
• Input: 0 → Output: 0

Solution Process

Step 1: Understanding the Problem

The first step in solving the problem is to accurately understand the given problem. The input is in the form of an integer, and what we need to do is separate this integer into its digits and sort them in descending order. After sorting the digits, we need to combine them back into an integer and return it.

Step 2: Extracting Each Digit

We can convert the integer into a string to easily access each digit. Each character in the string represents a digit of the original integer, and we can use this to extract the digits.

Step 3: Sorting the Digits

To sort the digits, we can use Kotlin’s built-in function. We can use the sortedDescending() function to sort the digits in descending order.

Step 4: Combining the Sorted Digits

We need to convert the sorted digits back into an integer as our final result. We concatenate each digit to form a single string and then convert it back into an integer to return it.

Step 5: Implementing the Final Code

fun solution(N: Int): Int {
    // Convert integer N to string
    val strN = N.toString()
    
    // Sort the digits in descending order
    val sortedDigits = strN.toCharArray().sortedDescending()
    
    // Combine the sorted digits back into a single string
    val resultStr = sortedDigits.joinToString("")
    
    // Convert string to integer and return
    return resultStr.toInt()
}
            

Example Test

To test the solution we have written, we will execute each test case.

fun main() {
    println(solution(118372)) // 873211
    println(solution(2143))   // 4321
    println(solution(0))      // 0
}
            

When the above code is executed, each integer is expected to be printed in descending order as intended.

Time Complexity and Space Complexity

The time complexity of the algorithm used here is O(d log d), where d is the number of digits. There is a logarithmic time complexity due to the sorting of the digits. The space complexity is O(d), as an array is used to store the digits.

Conclusion

Through this course, we learned how to leverage Kotlin’s powerful features and concise syntax for solving algorithm problems. The digit sorting problem serves as a good starting point that can be extended to other algorithm problems. It is important to practice solving more problems to familiarize oneself with various algorithms and data structures.

I hope this helps you in preparing for coding tests. Stay tuned for the next course!