Kotlin coding test course, binary search

Hello! In this course, we will explore the binary search algorithm, which commonly appears in job-related algorithm tests, and I will explain in detail how to solve problems using Kotlin. Binary search is an efficient algorithm for finding a specific value in a sorted array, with a time complexity of O(log n). In this course, we will solve problems using binary search and cover the problem-solving process in detail.

Problem Description

Problem: Given a sorted array and a specific value, find the index of that value. 
If the value does not exist in the array, it should return -1.

Input:
- arr: a sorted array of integers in ascending order
- target: the integer to be searched for

Output:
- The index of target (or -1 if it does not exist)

Input Example

arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
target = 5

Output Example

4

Overview of the Binary Search Algorithm

The binary search algorithm reduces the search range by splitting it into two branches based on conditions. The most common implementation of binary search includes the following steps.

1. Define the starting index and the ending index of the array.
2. Calculate the middle index.
3. Compare the middle value with the target value.
4. If the middle value is greater than the target, search the left half; if it is smaller, search the right half.
5. Repeat until the value is found or the search range becomes empty.

Kotlin Implementation

Now let’s implement binary search using Kotlin.

fun binarySearch(arr: IntArray, target: Int): Int {
    var left = 0
    var right = arr.size - 1

    while (left <= right) {
        val mid = left + (right - left) / 2

        if (arr[mid] == target) {
            return mid // return index if found
        } else if (arr[mid] < target) {
            left = mid + 1 // search right
        } else {
            right = mid - 1 // search left
        }
    }
    
    return -1 // return -1 if not found
}

Problem Solving Process

Now let’s apply the binary search algorithm using the example provided in the problem.

Example Analysis

arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
target = 5

The starting index for the binary search is 0, and the ending index is 8. Let’s calculate the middle index:

mid = left + (right - left) / 2
   = 0 + (8 - 0) / 2
   = 4

The value of arr[4] is 5. Since it matches the target value, we return index 4.

Performance Analysis

The time complexity of binary search is O(log n). This is because the search range is halved with each comparison; for example, if there are 8 elements in the array, it can be found in 3 comparisons. Binary search is very efficient for searching through a large amount of data.

Conclusion

In this course, we learned the concept of the binary search algorithm and how to implement it in Kotlin. This algorithm is useful for effectively finding specific values in a sorted array. When solving algorithm problems, it is important to understand the problem and choose the appropriate algorithm.

Practice solving more problems and improving your algorithm skills!