add 639

Author: fishercoder1534

README.md

@@ -28,6 +28,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |643|[Maximum Average Subarray I](https://leetcode.com/problems/maximum-average-subarray-i/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_643.java) | O(n) |O(1) | Easy |
 |642|[Design Search Autocomplete System](https://leetcode.com/problems/design-search-autocomplete-system/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_642.java) | O(n) |O(n) | Hard | Design
 |640|[Solve the Equation](https://leetcode.com/problems/solve-the-equation/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_640.java) | O(n) |O(n) | Medium |
+|639|[Decode Ways II](https://leetcode.com/problems/decode-ways-ii/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_639.java) | O(n) |O(n) | Hard| DP
 |638|[Shopping Offers](https://leetcode.com/problems/shopping-offers/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_638.java) | O(2^n) |O(n) | Medium | DP, DFS
 |637|[Average of Levels in Binary Tree](https://leetcode.com/problems/average-of-levels-in-binary-tree/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_637.java) | O(n) |O(1) | Easy | 
 |636|[Exclusive Time of Functions](https://leetcode.com/problems/exclusive-time-of-functions/)|[Solution](../master/src/main/java/com/fishercoder/solutions/_636.java) | O(n) |O(n/2) | Medium | Stack

src/main/java/com/fishercoder/solutions/_639.java

@@ -0,0 +1,63 @@
+package com.fishercoder.solutions;
+
+/**
+ * 639. Decode Ways II
+ *
+ * A message containing letters from A-Z is being encoded to numbers using the following mapping way:
+
+ 'A' -> 1
+ 'B' -> 2
+ ...
+ 'Z' -> 26
+ Beyond that, now the encoded string can also contain the character '*',
+ which can be treated as one of the numbers from 1 to 9.
+
+ Given the encoded message containing digits and the character '*',
+ return the total number of ways to decode it.
+
+ Also, since the answer may be very large, you should return the output mod 109 + 7.
+
+ Example 1:
+ Input: "*"
+ Output: 9
+ Explanation: The encoded message can be decoded to the string: "A", "B", "C", "D", "E", "F", "G", "H", "I".
+ Example 2:
+ Input: "1*"
+ Output: 9 + 9 = 18
+
+ Note:
+ The length of the input string will fit in range [1, 105].
+ The input string will only contain the character '*' and digits '0' - '9'.
+
+ */
+public class _639 {
+    /**reference: https://leetcode.com/articles/decode-ways-ii/#approach-2-dynamic-programming-accepted*/
+    int M = 1000000007;
+    public int numDecodings(String s) {
+        long[] dp = new long[s.length() + 1];
+        dp[0] = 1;
+        dp[1] = s.charAt(0) == '*' ? 9 : s.charAt(0) == '0' ? 0 : 1;
+        for (int i = 1; i < s.length(); i++) {
+            if (s.charAt(i) == '*') {
+                dp[i + 1] = 9 * dp[i];
+                if (s.charAt(i - 1) == '1') {
+                    dp[i + 1] = (dp[i + 1] + 9 * dp[i - 1]) % M;
+                } else if (s.charAt(i - 1) == '2') {
+                    dp[i + 1] = (dp[i + 1] + 6 * dp[i - 1]) % M;
+                } else if (s.charAt(i - 1) == '*') {
+                    dp[i + 1] = (dp[i + 1] + 15 * dp[i - 1]) % M;
+                }
+            } else {
+                dp[i + 1] = s.charAt(i) != '0' ? dp[i] : 0;
+                if (s.charAt(i - 1) == '1') {
+                    dp[i + 1] = (dp[i + 1] + dp[i - 1]) % M;
+                } else if (s.charAt(i - 1) == '2' && s.charAt(i) <= '6') {
+                    dp[i + 1] = (dp[i + 1] + dp[i - 1]) % M;
+                } else if (s.charAt(i - 1) == '*') {
+                    dp[i + 1] = (dp[i + 1] + (s.charAt(i) <= '6' ? 2 : 1) * dp[i - 1]) % M;
+                }
+            }
+        }
+        return (int) dp[s.length()];
+    }
+}