add: Decode Ways

Author: hijiangtao

README.md

@@ -31,6 +31,7 @@ This is the solutions collection of my LeetCode submissions, most of them are pr
 |69|[Sqrt(x)](https://leetcode.com/problems/sqrtx/) | [JavaScript](./src/sqrtx/res.js)|Easy|
 |73|[Set Matrix Zeroes](https://leetcode.com/problems/set-matrix-zeroes/) | [JavaScript](./src/set-matrix-zeroes/res.js)|Medium|
 |75|[Sort Colors](https://leetcode.com/problems/sort-colors/) | [JavaScript](./src/sort-colors/res.js)|Medium|
+|91|[Decode Ways](https://leetcode.com/problems/decode-ways/) | [JavaScript](./src/decode-ways/res.js)|Medium|
 |130|[Surrounded Regions](https://leetcode.com/problems/surrounded-regions/) | [JavaScript](./src/surrounded-regions/res.js)|Medium|
 |136|[Single Number](https://leetcode.com/problems/single-number/) | [JavaScript](./src/single-number/res.js)|Easy|
 |175|[Combine Two Tables](https://leetcode.com/problems/combine-two-tables/)| [SQL](./src/combine-two-tables/res.txt)|Easy|

src/decode-ways/res.js

@@ -0,0 +1,58 @@
+/**
+ * res.js
+ * @authors Joe Jiang (hijiangtao@gmail.com)
+ * @date    2017-04-06 18:43:29
+ * 
+ * A message containing letters from A-Z is being encoded to numbers using the following mapping:
+ * 'A' -> 1
+ * 'B' -> 2
+ * ...
+ * 'Z' -> 26
+ * 
+ * Given an encoded message containing digits, determine the total number of ways to decode it.
+ * 
+ * For example,
+ * Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
+ * 
+ * The number of ways decoding "12" is 2.
+ * 
+ * @param {string} s
+ * @return {number}
+ */
+let numDecodings = function(s) {
+   let len = s.length,
+    num = 0,
+    matrix = new Array(len);
+    
+    if (len===0) return len;
+    
+    for (let i=0; i<len; i++) {
+        matrix[i] = new Array(len);
+        if (s[i] !== '0') {
+            matrix[i][i] = 1;
+        } else {
+            matrix[i][i] = 0;
+        }
+    }
+    
+    let i=0;
+     for (let j=1; j<len; j++) {
+            let subOne = 0,
+                subTwo = 0,
+                subTwoStr = Number.parseInt(s.substring(j-1, j+1));
+                
+            if (s[j] !== '0') {
+                subOne = matrix[i][j-1];
+            }
+            if (subTwoStr > 9 && subTwoStr < 27) {
+                subTwo = j-i === 1? 1:matrix[i][j-2];
+            }
+            
+            if (subOne === 0 && subTwo === 0) return 0;
+            
+            matrix[i][j] = subOne + subTwo;
+            console.log(matrix[i][j]);
+        }
+    
+    return matrix[0][len-1];
+};