Swift Coding Test Course, Hacking Efficiently

Recently, many companies are conducting coding tests during the hiring process. In this article, I will cover a problem that can help you prepare for coding tests using the Swift programming language, and I will explain in detail how to solve that problem efficiently.

Problem: Sum of Two Numbers in an Array

Given an integer array nums and an integer target, you need to solve the problem of finding the two indices in the array such that their sum is equal to target. It is assumed that each input has exactly one solution, and you may not use the same element twice.

Input Format

• nums: [2, 7, 11, 15]
• target: 9

Output Format

[0, 1] (nums[0] + nums[1] = 2 + 7 = 9)

Problem Analysis

This problem is quite famous and can be solved in various ways. The most basic approach is to use a double loop, which has a time complexity of O(n^2). However, let’s explore a more efficient way.

Efficient Approach: Using Hash Map

You can use a method of iterating through the given array and storing each element in a hash map. By using a hash map, you can reduce the search time to O(1), thereby decreasing the overall time complexity to O(n).

Steps to Solve the Problem

1. Create an empty hash map.
2. Iterate through the array and calculate the difference between the current number current and target - current.
3. If target - current exists in the hash map, return the corresponding index and the current index.
4. Add the current number and its index to the hash map.

Swift Code

let nums = [2, 7, 11, 15]
let target = 9

func twoSum(nums: [Int], target: Int) -> [Int]? {
    var numDict = [Int: Int]()
    
    for (index, num) in nums.enumerated() {
        let complement = target - num
        
        if let complementIndex = numDict[complement] {
            // Return the two indices
            return [complementIndex, index]
        }
        
        // Add the current number and index to the hash map
        numDict[num] = index
    }
    
    // Return nil if no solution exists
    return nil
}

if let result = twoSum(nums: nums, target: target) {
    print(result) // [0, 1]
}
    

Result Verification

When you run the above code, it will print the result [0, 1]. This confirms that the sum of nums[0] and nums[1] is equal to target.

Optimization Considerations

This algorithm demonstrates an optimized approach to the given problem. Using the hash map allows us to solve the problem with an average time complexity of O(n). However, in the worst case, performance can degrade due to hash collisions, so it’s essential to use an appropriate hash function.

Conclusion

In this article, we explored how to solve coding test problems using Swift. The hash map approach can be applied in various situations and can significantly help in writing efficient code. I encourage you to continue solving various algorithm problems to improve your skills.

There will be more courses covering various algorithm problems and their solutions, so please stay tuned!