Add solution #13

Author: JoshCrozier

README.md

@@ -14,6 +14,7 @@
 8|[String to Integer (atoi)](./0008-string-to-integer-atoi.js)|Medium|
 10|[Regular Expression Matching](./0010-regular-expression-matching.js)|Hard|
 12|[Integer to Roman](./0012-integer-to-roman.js)|Medium|
+13|[Roman to Integer](./0013-roman-to-integer.js)|Easy|
 14|[Longest Common Prefix](./0014-longest-common-prefix.js)|Easy|
 17|[Letter Combinations of a Phone Number](./0017-letter-combinations-of-a-phone-number.js)|Medium|
 27|[Remove Element](./0027-remove-element.js)|Easy|

solutions/0013-roman-to-integer.js

@@ -0,0 +1,45 @@
+/**
+ * 13. Roman to Integer
+ * https://leetcode.com/problems/roman-to-integer/
+ * Difficulty: 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.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var romanToInt = function(s) {
+  const map = { M: 1000, D: 500, C: 100, L: 50, X: 10, V: 5, I: 1 };
+
+  return s.split('').reduce((result, v, i) => {
+    result += map[v] >= map[s[i + 1]] || s.length === i + 1
+      ? map[v]
+      : -1 * map[v];
+    return result;
+  }, 0);
+};