Add new problem - Roman to Integer.

Author: swiftyfinch

README.md

@@ -1,6 +1,6 @@
 # Leetcode
 
-### 📖 [Top 100 Interview Questions](https://leetcode.com/problemset/top-interview-questions/) (9/100 Solved) ✅
+### 📖 [Top 100 Interview Questions](https://leetcode.com/problemset/top-interview-questions/) (10/100 Solved) ✅
 
 1. [Two Sum](<Top 100 Interview Questions/Two Sum>) (Easy)
 2. [Add Two Numbers](<Top 100 Interview Questions/Add Two Numbers>) (Medium)
@@ -11,3 +11,4 @@
 7. [String to Integer (atoi)](<Top 100 Interview Questions/String to Integer (atoi)>) (Medium)
 8. [Regular Expression Matching](<Top 100 Interview Questions/Regular Expression Matching>) (Hard)
 9. [Container With Most Water](<Top 100 Interview Questions/Container With Most Water>) (Medium)
+10. [Roman to Integer](<Top 100 Interview Questions/Roman to Integer>) (Easy)

Top 100 Interview Questions/Roman to Integer/README.md

@@ -0,0 +1,67 @@
+# [Roman to Integer](https://leetcode.com/problems/roman-to-integer/) (Easy)
+
+Roman numerals are represented by seven different symbols: `I`, `V`, `X`, `L`, `C`, `D` and `M`.
+
+```
+Symbol       Value
+I             1
+V             5
+X             10
+L             50
+C             100
+D             500
+M             1000
+```
+
+For example, `2` is written as `II` in Roman numeral, just two one's added together. `12` is written as `XII`, which is simply `X + II`. The number `27` is written as `XXVII`, which is `XX + V + II`.
+
+Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not `IIII`. Instead, the number four is written as `IV`. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as `IX`. There are six instances where subtraction is used:
+
+* `I` can be placed before `V` (5) and `X` (10) to make 4 and 9. 
+* `X` can be placed before `L` (50) and `C` (100) to make 40 and 90. 
+* `C` can be placed before `D` (500) and `M` (1000) to make 400 and 900.
+
+Given a roman numeral, convert it to an integer.
+
+### Example 1:
+
+```
+Input: s = "III"
+Output: 3
+```
+
+### Example 2:
+
+```
+Input: s = "IV"
+Output: 4
+```
+
+### Example 3:
+
+```
+Input: s = "IX"
+Output: 9
+```
+
+### Example 4:
+
+```
+Input: s = "LVIII"
+Output: 58
+Explanation: L = 50, V= 5, III = 3.
+```
+
+### Example 5:
+
+```
+Input: s = "MCMXCIV"
+Output: 1994
+Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
+```
+
+### Constraints:
+
+* `1 <= s.length <= 15`
+* `s` contains only the characters `('I', 'V', 'X', 'L', 'C', 'D', 'M')`.
+* It is **guaranteed** that `s` is a valid roman numeral in the range `[1, 3999]`.

Top 100 Interview Questions/Roman to Integer/RomanToInteger.swift

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+final class Solution {
+    func romanToInt(_ string: String) -> Int {
+        let map: [Character: Int] = [
+            "I": 1, "V": 5, "X": 10, "L": 50, "C": 100, "D": 500, "M": 1000
+        ]
+        
+        var number = 0
+        var index = string.startIndex
+        while index != string.endIndex {
+            guard let char = map[string[index]] else { return -1 }
+            
+            let next = string.index(after: index)
+            if next < string.endIndex, let nextChar = map[string[next]], char < nextChar {
+                number += nextChar - char
+                index = string.index(after: next)
+            } else {
+                number += char
+                index = next
+            }
+        }
+        return number
+    }
+}