Kotlin coding test course, which algorithm to use

In this article, we will introduce an algorithm problem that is frequently encountered in coding tests using Kotlin and discuss the process of solving it in detail. Coding tests have become an important means of assessing various problem-solving abilities, and understanding and utilizing specific algorithms is key to succeeding in them.

Problem Description

The following is a common problem that could appear in coding tests.

Problem: Sum of Two Numbers

You are given an integer array numbers and an integer target. Write a function that returns the indices of the two numbers such that they add up to the target. If there are no two numbers that add up to the target, return -1.

Example Input

numbers = [2, 7, 11, 15]
target = 9

Example Output

[0, 1]

Problem Solving Approach

To solve this problem, we can use the following approaches.

1. Brute Force

The simplest and most intuitive approach is to use two nested loops to check all possible combinations of two numbers. However, this method has a time complexity of O(n^2), making it very inefficient.

2. Using a HashMap

A more efficient approach is to utilize a HashMap (or dictionary). This method has a time complexity of O(n), solving the problem with only one loop.

  • While storing each number in the HashMap, check if the number that, when added to the current number, equals the target is in the HashMap.
  • The HashMap will store each number and its index.

Implementing in Kotlin

Now, let’s implement the previously mentioned HashMap approach in Kotlin.

fun twoSum(numbers: IntArray, target: Int): IntArray {
    val map = mutableMapOf()

    for (i in numbers.indices) {
        val complement = target - numbers[i]
        // Check if complement is in the map
        if (map.containsKey(complement)) {
            // Return the indices found
            return intArrayOf(map[complement]!!, i)
        }
        // Store the current number and index
        map[numbers[i]] = i
    }
    // Return -1 if no two numbers match
    return intArrayOf(-1)
}

Code Explanation

Let’s take a look at each part of the code.

  • fun twoSum(numbers: IntArray, target: Int): IntArray: The function takes an integer array and a target integer as parameters.
  • val map = mutableMapOf(): Creates a HashMap to store each number and its index.
  • for (i in numbers.indices): Iterates through the given array to retrieve the index of each element.
  • val complement = target - numbers[i]: Calculates the other number that would add up to the target.
  • if (map.containsKey(complement)): Checks if the HashMap contains the ‘complement’.
  • return intArrayOf(map[complement]!!, i): If the ‘complement’ exists, returns the indices of the two numbers as an array.
  • Finally, adds the current number and its index to the HashMap and continues the iteration.

Time Complexity Analysis

The time complexity of this algorithm is O(n). This is because it traverses the array once while accessing the HashMap, which has constant time complexity. Therefore, the execution time increases proportionally with the size of the input array. The space complexity is also O(n) because, in the worst case, all numbers could be stored in the HashMap.

Conclusion

In this post, we have solved a Kotlin algorithm problem. We looked at how to efficiently solve problems using a HashMap. Such problems frequently appear in coding tests, so it’s important to practice various problems to develop a sense for them. Additionally, learning and utilizing Kotlin’s various features will greatly help in preparing for coding tests.

Final Thoughts

In the next post, we will discuss another algorithm topic. It is important to have a good understanding of various algorithms and utilize them, so I encourage you to improve your skills through consistent practice. If you have any questions or comments, please leave them below.