Kotlin Coding Test Course, Quick Sort

Quick Sort is a very efficient sorting algorithm, especially with an average time complexity of O(n log n) and a space complexity of O(log n). In this article, we will take a detailed look at the concept of Quick Sort, how to implement it in Kotlin, algorithm problems, and the problem-solving process.

1. Concept of Quick Sort

Quick Sort is a type of divide and conquer algorithm that operates in the following steps:

Select one element from the given array as a pivot. Usually, the first element, the last element, or the middle element of the array is chosen.
Split the array into two sub-arrays based on the pivot. Elements smaller than the pivot are positioned on the left, and larger elements on the right.
Recursively apply Quick Sort on the left and right sub-arrays.

This process is repeated until the array cannot be divided any further. The greatest advantage of Quick Sort is that it is very fast on average, and it is an in-place sorting algorithm. Therefore, it uses very little additional memory.

2. Example of Quick Sort in Action

Assume the array to be sorted is [3, 6, 8, 10, 1, 2, 1]. Below is the step-by-step process of Quick Sort.

Step 1: Choose a Pivot

Pivot: 1

Step 2: Split the Array

[1, 2, 1, 10, 6, 8, 3]

Step 3: Recursive Call

The left array [1, 1] is already sorted.
Apply Quick Sort again on the right array [10, 6, 8, 3].

Step 4: Final Sort

[1, 1, 2, 3, 6, 8, 10]

3. Algorithm Problem

Now, let’s solve a problem that returns a sorted array using Quick Sort.

Problem: Sort the given integer array using Quick Sort.

Input: [10, 7, 8, 9, 1, 5]

Output: [1, 5, 7, 8, 9, 10]

4. Kotlin Implementation

Below is the Kotlin code to solve the above problem.


fun quickSort(arr: IntArray, low: Int, high: Int) {
    if (low < high) {
        val pi = partition(arr, low, high)
        quickSort(arr, low, pi - 1)
        quickSort(arr, pi + 1, high)
    }
}

fun partition(arr: IntArray, low: Int, high: Int): Int {
    val pivot = arr[high]
    var i = (low - 1)

    for (j in low until high) {
        if (arr[j] <= pivot) {
            i++
            val temp = arr[i]
            arr[i] = arr[j]
            arr[j] = temp
        }
    }

    val temp = arr[i + 1]
    arr[i + 1] = arr[high]
    arr[high] = temp
    return i + 1
}

fun main() {
    val arr = intArrayOf(10, 7, 8, 9, 1, 5)
    quickSort(arr, 0, arr.size - 1)
    println("Sorted array: ${arr.joinToString(", ")}")
}

5. Code Explanation

5.1 quickSort Function

The quickSort function is called recursively, taking the starting and ending indices of the array as arguments. If the starting index is less than the ending index, the partition function is called to find the position of the pivot, based on which the array is rearranged.

5.2 partition Function

The partition function is responsible for dividing the array based on the pivot. It checks all elements and moves the elements smaller than the pivot to the left array. Finally, it places the pivot in the correct position and returns that index.

6. Complexity Analysis

The time complexity of Quick Sort is as follows:

Best case: O(n log n)
Average case: O(n log n)
Worst case: O(n²) – This can occur if an already sorted array is provided as input.

The space complexity is O(log n). This is due to stack memory usage from recursive calls.

7. Conclusion

In this article, we explored how to implement the Quick Sort algorithm in Kotlin. Quick Sort is widely used due to its efficiency and simplicity. It often appears in situations like coding tests, so a fundamental understanding and practice are very important.

8. Additional Practice Problems

Below are some additional practice problems where Quick Sort can be applied:

Implement a method to find the k-th smallest element in the array.
Implement Quick Sort non-recursively.
Solve the problem of merging two sorted arrays.

Quick Sort is a great way to learn various important concepts in algorithms and data structures. Continue practicing and expand your understanding into deep knowledge.