Add two sum problem.

Author: swiftyfinch

.gitignore

@@ -0,0 +1 @@
+.DS_Store

README.md

@@ -1 +1,5 @@
-# Leetcode
+# Leetcode
+
+### 📖 [Top 100 Interview Questions](https://leetcode.com/problemset/top-interview-questions/) 1/145 Solved
+
+1. [Two Sum](<Top 100 Interview Questions/Two Sum>)

Top 100 Interview Questions/Two Sum/README.md

@@ -0,0 +1,36 @@
+# [Two Sum](https://leetcode.com/problems/two-sum/)
+
+Given an array of integers `nums` and an integer `target`, return *indices of the two numbers such that they add up to `target`*.
+
+You may assume that each input would have **exactly one solution**, and you may not use the same element twice.
+
+You can return the answer in any order.
+
+##### Example 1:
+
+```
+Input: nums = [2,7,11,15], target = 9
+Output: [0,1]
+Output: Because nums[0] + nums[1] == 9, we return [0, 1].
+```
+
+##### Example 2:
+
+```
+Input: nums = [3,2,4], target = 6
+Output: [1,2]
+```
+
+##### Example 3:
+
+```
+Input: nums = [3,3], target = 6
+Output: [0,1]
+```
+ 
+##### Constraints:
+
+* `2 <= nums.length <= 10³`
+* `-10⁹ <= nums[i] <= 10⁹`
+* `-10⁹ <= target <= 10⁹`
+* **Only one valid answer exists**.

Top 100 Interview Questions/Two Sum/TwoSum.swift

@@ -0,0 +1,13 @@
+final class Solution {
+    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
+        var map: [Int: Int] = [:]
+        for (index, value) in nums.enumerated() {
+            let remainder = target - value
+            if let rightIndex = map[remainder] {
+                return [index, rightIndex]
+            }
+            map[value] = index
+        }
+        return []
+    }
+}