Add solution #91

Author: JoshCrozier

README.md

@@ -96,6 +96,7 @@
 88|[Merge Sorted Array](./0088-merge-sorted-array.js)|Easy|
 89|[Gray Code](./0089-gray-code.js)|Medium|
 90|[Subsets II](./0090-subsets-ii.js)|Medium|
+91|[Decode Ways](./0091-decode-ways.js)|Medium|
 92|[Reverse Linked List II](./0092-reverse-linked-list-ii.js)|Medium|
 93|[Restore IP Addresses](./0093-restore-ip-addresses.js)|Medium|
 94|[Binary Tree Inorder Traversal](./0094-binary-tree-inorder-traversal.js)|Easy|

solutions/0091-decode-ways.js

@@ -0,0 +1,57 @@
+/**
+ * 91. Decode Ways
+ * https://leetcode.com/problems/decode-ways/
+ * Difficulty: Medium
+ *
+ * You have intercepted a secret message encoded as a string of numbers. The message
+ * is decoded via the following mapping:
+ * "1" -> 'A'
+ * "2" -> 'B'
+ * ...
+ * "25" -> 'Y'
+ * "26" -> 'Z'
+ *
+ * However, while decoding the message, you realize that there are many different
+ * ways you can decode the message because some codes are contained in other codes
+ * ("2" and "5" vs "25").
+ *
+ * For example, "11106" can be decoded into:
+ * - "AAJF" with the grouping (1, 1, 10, 6)
+ * - "KJF" with the grouping (11, 10, 6)
+ * - The grouping (1, 11, 06) is invalid because "06" is not a valid code (only "6"
+ *   is valid).
+ *
+ * Note: there may be strings that are impossible to decode.
+ *
+ * Given a string s containing only digits, return the number of ways to decode it.
+ * If the entire string cannot be decoded in any valid way, return 0.
+ *
+ * The test cases are generated so that the answer fits in a 32-bit integer.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var numDecodings = function(s) {
+  if (s == null || s.length === 0) return 0;
+  if (s[0] === '0') return 0;
+
+  const group = new Array(s.length + 1).fill(0);
+  group[0] = 1;
+  group[1] = 1;
+
+  for (let i = 2; i <= s.length; i++) {
+    const a = Number(s.slice(i - 1, i));
+    if (a >= 1 && a <= 9) {
+      group[i] += group[i - 1];
+    }
+
+    const b = Number(s.slice(i - 2, i));
+    if (b >= 10 && b <= 26) {
+      group[i] += group[i - 2];
+    }
+  }
+
+  return group[s.length];
+};