solves decode ways

Author: anishLearnsToCode

README.md

@@ -81,6 +81,7 @@
 |  88  | [Merge Sorted Array](https://leetcode.com/problems/merge-sorted-array)                                                                                    |                                  [![Java](assets/java.png)](src/MergeSortedArray.java) [![Python](assets/python.png)](python/merge_sorted_array.py)                                  |                                                                                                                                   |
 |  89  | [Gray Code](https://leetcode.com/problems/gray-code)                                                                                                      |                                                                    [![Java](assets/java.png)](src/GrayCode.java)                                                                     |                                                                                                                                   |
 |  90  | [Subsets II](https://leetcode.com/problems/subsets-ii)                                                                                                    |                                                                    [![Java](assets/java.png)](src/SubsetsII.java)                                                                    |                                                                                                                                   |
+|  91  | [Decode Ways](https://leetcode.com/problems/decode-ways)                                                                                                  |                                                                   [![Java](assets/java.png)](src/DecodeWays.java)                                                                    |                                                                                                                                   |
 |  94  | [Binary Tree Inorder Traversal](https://leetcode.com/problems/binary-tree-inorder-traversal/)                                                             |                       [![Java](assets/java.png)](src/BinaryTreeInorderTraversal.java) [![Python](assets/python.png)](python/binary_tree_inorder_traversal.py)                        |                                                                                                                                   |
 | 100  | [Same Tree](https://leetcode.com/problems/same-tree)                                                                                                      |                                          [![Java](assets/java.png)](src/SameTree.java) [![Python](assets/python.png)](python/same_tree.py)                                           |                                                                                                                                   |
 | 101  | [Symmetric Tree](https://leetcode.com/problems/symmetric-tree)                                                                                            |                                     [![Java](assets/java.png)](src/SymmetricTree.java) [![Python](assets/python.png)](python/symmetric_tree.py)                                      |                                                                                                                                   |

src/DecodeWays.java

@@ -0,0 +1,27 @@
+// https://leetcode.com/problems/decode-ways
+// T: O(n)
+// S: O(1)
+
+public class DecodeWays {
+    private static final char ZERO = '0';
+
+    public int numDecodings(String s) {
+        int previous = 1, current, temp;
+        current = s.charAt(s.length() - 1) > ZERO ? 1 : 0;
+        for (int i = s.length() - 2 ; i >= 0 ; i--) {
+            if (s.charAt(i) == ZERO) {
+                previous = current;
+                current = 0;
+            } else {
+                temp = current;
+                current += isValidAlphabet(s.charAt(i), s.charAt(i + 1)) ? previous : 0;
+                previous = temp;
+            }
+        }
+        return current;
+    }
+
+    private boolean isValidAlphabet(char a, char b) {
+        return (a - ZERO) * 10 + (b - ZERO) <= 26;
+    }
+}