This is a simple solution to the famous "2 Sum" problem. If you like feel free to download and tinker with the XCode Playground file from my Github repository link https://github.com/KSJain/2021-LeetCode-01_Two_Sum

I would be perodically keep solving as many questions as I could, to keep everyday interesting. I miss school days.

As of posting date the Stats are as following

Runtime: 36 ms, faster than 73.77% of Swift online submissions for Two Sum. Memory Usage: 14.4 MB, less than 39.16% of Swift online submissions for Two Sum. Big-O Time Complexity: O(n)

Feel free to comment and/or improve. I would like to learn and improve alongside my peers : )

Happy Coding

func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
    guard nums.count > 1 else { return [-1, -1] }
    
    var directory = [Int: Int]()
    
    for num in nums.enumerated() {
        let counter = target - num.element
        if let otherIndex = directory[counter] {
            return [min(num.offset, otherIndex), max(num.offset, otherIndex)]
        } else {
            directory[num.element] = num.offset
        }
    }
    
    return [-1, -1]
}

Code in a nutshell:

1. guard checks if there are atleast two numbers in the array input, if not eary termination with placeholder result of invalid indices [-1,-1]
2. Initalize a local directory to store indices corresponding for discovered values in array input (key: number, value: index)
3. Uses swift for-in loop, with nums.enumerated(),which gets captured in num variable as an (element, offset) tuple
4. counter value is determined subtracting current element value from the target value provided to the function
5. directory is queried for key value : counter 5a.If the value exists (not nil) it is captured in otherIndex variable and returned in as a pair*. The function terminates here 5b. If the value didnt exist (nil value) simply store it for the future usage
6. Finally if the list is exhausted - meaning no pair was found - return [-1, -1] as a placeholder

Ideally one would return a nil or Result type instead of placeholder value if there was an error. But this is just an easy code challange :p

If you like - come to my page https://codinginferno.com/ for more fun learning. I love to learn while sharing, so feel free to ask.