The typical two sum solution

TwoSum.swift

// Create a dictionary with the key the index of each item in the array.
// The value is the difference between the array's value and the target.
// Now if we want to find the first solution that contains sum, just loop
// through the array and check the dictionary for other numbers who match the difference.

class Solution {
    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
        var dict = [Int:Int]()
        
        for (i, val) in nums.enumerated() {
            
            let differenceBetweenTwoNumbers = target-val
            
            if let difference = hashTable[differenceBetweenTwoNumbers] {
                return [difference, i]
            }
            dict[val] = i
        }
        return [0,0] //Should not hit
    }
}