Kotlin coding test course, radix sort

Hello! Today we will discuss Radix Sort, which is essential for job seekers and those preparing for coding tests. Radix sort is one of the most efficient algorithms for sorting integers, particularly shining in large datasets.

What is Radix Sort?

Radix Sort is a type of non-comparative sorting algorithm that processes values by separating them by individual digits. This method is especially effective for integer data and is a stable sort. Unlike most sorting algorithms, Radix Sort is not comparison-based, resulting in a time complexity of O(nk), where n is the number of elements to be sorted, and k is the number of digits in the largest number.

Problem Description

Problem: Sort the given list of integers in ascending order using the Radix Sort algorithm.

You need to output the sorted result using Radix Sort when given the following input:

Input: [170, 45, 75, 90, 802, 24, 2, 66]

Output: [2, 24, 45, 66, 75, 90, 170, 802]

Understanding the Radix Sort Algorithm

The main idea of Radix Sort is to repeatedly use Counting Sort based on each digit. Let’s look at the steps of Radix Sort.

Step 1: Separation of Digits

Each number is processed separately based on its digits (those in the 1s, 10s, 100s place, etc.). When processing each digit, Counting Sort is used to sort these numbers.

Step 2: Counting Sort

Counting Sort is a method of sorting the input list based on a specific digit. Because it is stable, it can maintain the relative order established earlier.

Step 3: Repeat

Starting from the smallest place value, repeat sorting to the largest place value to obtain the final sorted result.

Implementation of Radix Sort (Kotlin Example)

Here is a code implementing Radix Sort using Kotlin:

fun countingSort(array: IntArray, place: Int) {
    val size = array.size
    val output = IntArray(size)
    val count = IntArray(10)

    for (i in 0 until size) {
        count[(array[i] / place) % 10]++
    }

    for (i in 1 until 10) {
        count[i] += count[i - 1]
    }

    for (i in size - 1 downTo 0) {
        output[count[(array[i] / place) % 10] - 1] = array[i]
        count[(array[i] / place) % 10]--
    }

    for (i in 0 until size) {
        array[i] = output[i]
    }
}

fun radixSort(array: IntArray) {
    val max = array.maxOrNull() ?: return

    for (place in 1 until max + 1 step 10) {
        countingSort(array, place)
    }
}

In the code above, the countingSort function is defined to perform Counting Sort first. Then, the radixSort function implements the sorting by iterating over the digits based on the maximum value of the given array.

Code Explanation

• The countingSort function performs sorting based on the given digit.
• place indicates the current digit being sorted, starting from the 1s place and increasing by 10.
• The count array is used to count the occurrences of each digit (0-9).
• Finally, the sorted result is stored in the output array, and the input array is updated.

Time Complexity of Radix Sort

The time complexity of Radix Sort is O(nk), where k is the number of digits and n is the size of the array. Most other comparison-based sorting algorithms have an average time complexity of O(n log n), making Radix Sort perform much faster, especially when the input data is large with fewer digits.

Limitations of Radix Sort

Radix Sort has several limitations:

• Suitable for integer data: This algorithm is specialized for integer data and requires a modified version for strings or floating-point numbers.
• Range Limitation: For integers with large value ranges, memory usage may increase, so caution is needed.
• Memory Usage: It may require more memory since it generates temporary arrays at intermediate stages.

Conclusion

In this tutorial, we have explored Radix Sort in detail. Radix Sort is a stable and highly effective sorting algorithm in specific situations. Understanding Radix Sort and applying it correctly can be very helpful in coding tests. Next, we will cover other sorting algorithms as well. Thank you!