Coding tests are one of the challenges that many developers face. In Swift coding tests, it is essential to understand and utilize various algorithms and data structures well. In this article, we will illustrate using a real algorithm problem and explore the process of solving the problem in detail.
Problem: Two Sum
Description
Given an integer array and an integer target value, write a program to select two numbers that add up to the target value. Return the indices of the selected two numbers. Each element of the array is unique.
Input
- Integer array
nums: [2, 7, 11, 15] - Integer
target: 9
Output
An array representing the indices of the selected two numbers. For example, return [0, 1].
Example
Input: nums = [2, 7, 11, 15], target = 9
Output: [0, 1]
Problem Solving Process
1. Understanding the Problem
This problem is about finding the sum of two numbers. The most critical point is that we need to find the indices of the two numbers. Therefore, we must think about how to find them efficiently rather than checking all possible combinations.
2. Choosing an Algorithm
There are several methods to solve this problem, but the most efficient way is to use a hashmap. By using a hashmap, we can check each number one by one and look for the value subtracted from the target value in the hashmap.
3. Algorithm Explanation
- Initialize the hashmap.
- Traverse the array and check each number.
- Check if the value subtracted from the target is present in the hashmap.
- If it exists, return the index of that number and the current index.
- If it does not exist, add the current number to the hashmap.
4. Code Implementation
Now, let’s write the code in Swift based on the above algorithm.
func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
var numDict = [Int: Int]() // Hashmap
for (index, num) in nums.enumerated() {
let complement = target - num // Subtract current number from target
if let complementIndex = numDict[complement] { // Search in hashmap
return [complementIndex, index] // Return indices
}
numDict[num] = index // Add current number to hashmap
}
return [] // Return empty array if no value found
}
// Example usage
let nums = [2, 7, 11, 15]
let target = 9
let result = twoSum(nums, target)
print(result) // Outputs: [0, 1]
5. Time Complexity Analysis
The time complexity of this algorithm is O(n). This is because we traverse the array once while adding values to the hashmap and checking existing values. The space complexity is O(n), which is proportional to the size of the hashmap.
Conclusion
What is important in Swift coding tests is to understand the problem and choose the appropriate algorithm to solve it efficiently. Through this problem, we learned an approach using a hashmap. Practice various algorithms and data structures to prepare well for coding tests.