Today, we will discuss how to prepare for coding tests using Kotlin. In particular, we will focus on arrays and lists, and aim to enhance our understanding through practical problems.
1. Problem Definition
Below is a basic algorithm problem.
Problem: Sum of Two Numbers
Given an integer array nums and an integer target, write a function that returns the indices of the two elements in the nums array that add up to the target. If no such indices exist, return -1.
For example, here is a case:
Input: nums = [2, 7, 11, 15], target = 9
Output: [0, 1]2. Problem-Solving Process
To solve this problem, we need to consider the following steps.
2.1. Problem Analysis
The problem is to add two numbers in an array to create a specific value and find the indices of those numbers. Thus, there are ways to check all combinations using a standard double loop, as well as using a hashmap for better performance.
2.2. Approach
Let’s first consider the method using a double loop. This method checks combinations with each element in the array serving as the basis. However, this method has a time complexity of O(n^2) and is thus inefficient.
Instead, we can consider using a hashmap. With a hashmap, we can efficiently store the index of each element. This allows us to reduce the time complexity to O(n).
2.3. Solution Using Hashmap
The process for the solution using a hashmap is as follows:
- Traverse the integer array
nums. - For each element, calculate
target - nums[i]. - Check if the calculated value exists in the hashmap. If it does, return the index of that element.
- If it does not exist, store the element and its index in the hashmap.
2.4. Kotlin Code Implementation
Let’s write the code. The code below uses the above method to solve the problem.
fun twoSum(nums: IntArray, target: Int): IntArray {
val map = mutableMapOf()
for (i in nums.indices) {
val complement = target - nums[i]
if (map.containsKey(complement)) {
return intArrayOf(map[complement]!!, i)
}
map[nums[i]] = i
}
return intArrayOf(-1) // No solution found
} 3. Implementation Results
Now, implementing the above code, we can obtain the following result for the given example:
val nums = intArrayOf(2, 7, 11, 15)
val target = 9
val result = twoSum(nums, target)
println(result.joinToString(", ")) // Output: 0, 1When the above code is executed, it will output the correct indices [0, 1].
4. Problem Variation
We can modify the above problem to consider cases where the same number can be used multiple times. For example, we can modify the problem to find all indices that can be used to create the target if the elements of the integer array nums can be duplicated.
In this case, we should also consider possible alternative approaches using a hashmap.
5. Conclusion
In this tutorial, we solved the basic algorithm problem ‘Sum of Two Numbers’ using Kotlin, which involves handling arrays and lists. We learned how to solve the problem through an effective approach using a hashmap. Arrays and lists are very important data structures, and by practicing various variant problems, you can achieve high scores in coding tests.
We will continue to introduce problem-solving processes and methods for various algorithm problems, so please stay tuned. Thank you!