Swift Coding Test Course, Preparing for Resignation

1. Problem Description

As you prepare for resignation, it is important to develop your ability to solve algorithm problems using the Swift language. The following is one of the frequently asked questions in Swift coding tests.

Problem: Sum of Two Numbers in an Array

Given an integer array nums and an integer target, write a function that returns the indices of the two numbers in nums that add up to target. It is assumed that there is exactly one solution for each input, and you cannot use the same element twice. The order of the returned indices does not matter.

Example

    Input: nums = [2, 7, 11, 15], target = 9
    Output: [0, 1]  // nums[0] + nums[1] == 9

2. Understanding the Problem and Approach

This problem can be approached as follows:

  • A method using a double loop to check all possible two-number combinations
  • A method using a hash map to check the requirements while traversing only once

To optimally meet the requirements, I will choose to implement the method using a hash map. This implementation has a time complexity of O(n) and a space complexity of O(n).

3. Swift Implementation

3.1. Setting Up Required Libraries and Basic Structure

    import Foundation

    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        var numDict = [Int: Int]()
        // The contents of the function will go here.
    }

3.2. Implementation Using Hash Map

The following code implements the functionality of finding the indices of two numbers using a hash map:

    import Foundation

    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 [complementIndex, index]
            }
            numDict[num] = index
        }
        return []
    }

4. Function Explanation

The above twoSum function performs the following tasks:

  1. Iterates through the given array nums and stores each integer in the hash map.
  2. Calculates the value by subtracting the number from target for each integer. This is called the complement.
  3. Checks if the complement exists in the hash map. If it does, adds that index to the result array.
  4. If it does not exist, adds the current number and its index to the hash map.

5. Test Cases

Let’s write several cases to test the implemented function.

    let nums1 = [2, 7, 11, 15]
    let target1 = 9
    print(twoSum(nums1, target1))  // [0, 1]

    let nums2 = [3, 2, 4]
    let target2 = 6
    print(twoSum(nums2, target2))  // [1, 2]

    let nums3 = [3, 3]
    let target3 = 6
    print(twoSum(nums3, target3))  // [0, 1]

6. Complexity Analysis

The time complexity of the twoSum function is O(n) because it traverses the array once. The space complexity is O(n) because the hash map can hold up to n elements.

7. Conclusion and Further Learning

The problem of the sum of two numbers in an array is a very important problem in preparing for coding tests using Swift. This problem helps to understand and utilize the efficiency of hash maps. Let’s focus on enhancing our skills by solving various algorithm problems in the future.

8. References