Updated solution 1 for clarity.

Two-Sum Problem/README.markdown

@@ -1,24 +1,24 @@
 # Two-Sum Problem
 
-You're given an array `a` with numbers. Write an algorithm that checks if there are any two entries in the array that add up to a given number `k`. In other words, is there any `a[i] + a[j] == k`?
+You're given an array of numbers. Write an algorithm that checks if there are any two elements in the array that add up to a given number.
 
 There are a variety of solutions to this problem (some better than others). The following solutions both run in **O(n)** time.
 
 # Solution 1
 
-This solution uses a dictionary to store differences between each element in the array and the sum `k` that we're looking for. The dictionary also stores the indices of each element.
+This solution uses a dictionary to store differences between each element in the array and the sum that we're looking for. The dictionary also stores the indices of each element.
 
-With this approach, each key in the dictionary corresponds to a new target value. If one of the successive numbers from the array is equal to one of the dictionary's keys, then we know there exist two numbers that sum to `k`.
+With this approach, each key in the dictionary corresponds to a new target value. If one of the successive numbers from the array is equal to one of the dictionary's keys, then we know there exist two numbers that sum.
 
 ```swift
-func twoSumProblem(_ a: [Int], k: Int) -> ((Int, Int))? {
-  var dict = [Int: Int]()
-
-  for i in 0 ..< a.count {
-    if let newK = dict[a[i]] {
-      return (newK, i)
+func twoSumProblem(_ numbers: [Int], target: Int) -> (Int, Int)? {
+  var dict: [Int: Int] = [:]
+  
+  for (index, number) in numbers.enumerated() {
+    if let otherIndex = dict[number] {
+      return (index, otherIndex)
     } else {
-      dict[k - a[i]] =  i
+      dict[target - number] = index
     }
   }
 

Two-Sum Problem/Solution 1/2Sum.playground/Contents.swift

@@ -4,26 +4,21 @@
 print("Hello, Swift 4!")
 #endif
 
-func twoSumProblem(_ a: [Int], k: Int) -> ((Int, Int))? {
-  var map = [Int: Int]()
-
-  for i in 0 ..< a.count {
-    if let newK = map[a[i]] {
-      return (newK, i)
+func twoSum(_ numbers: [Int], target: Int) -> (Int, Int)? {
+  var dict: [Int: Int] = [:]
+  
+  for (index, number) in numbers.enumerated() {
+    if let otherIndex = dict[number] {
+      return (index, otherIndex)
     } else {
-      map[k - a[i]] =  i
+      dict[target - number] = index
     }
   }
+
   return nil
 }
 
-let a = [7, 100, 2, 21, 12, 10, 22, 14, 3, 4, 8, 4, 9]
-if let (i, j) = twoSumProblem(a, k: 33) {
-  i            // 3
-  a[i]         // 21
-  j            // 4
-  a[j]         // 12
-  a[i] + a[j]  // 33
-}
+let numbers = [3, 2, 4]
+let target = 6
 
-twoSumProblem(a, k: 37)  // nil
+twoSum(numbers, target: target) // expected output: indices 2 and 1