Merge pull request #716 from Karnika06/Devincept7

Added Decode Ways II (LeetCode) problem

Author: Saviour1001

Strings/DecodeWaysII.java

@@ -0,0 +1,110 @@
+/*
+ * Implementation of Decode Ways II problem from LeetCode(Hard)
+ * 
+ * Problem link: https://leetcode.com/problems/decode-ways-ii/
+ * 
+ * Time Complexity: O(n)
+ * Space Complexity: O(n)
+ */
+
+import java.util.*;
+import java.io.*;
+
+class DecodeWaysII 
+{
+    public int numDecodings(String s) 
+    {
+        long[] dp=new long[s.length()+1];
+        dp[0]=1;
+        dp[1]= decodeChar(s.charAt(0));
+        for(int i=2;i<=s.length();i++)
+        {
+            
+            char prev=s.charAt(i-2);
+            char curr=s.charAt(i-1);
+            
+            //single character decoding
+            dp[i] += dp[i-1]*decodeChar(curr);
+            
+            //double character decoding
+            dp[i] += dp[i-2]*decodeTwoChars(prev, curr);
+            
+            dp[i] = dp[i]%1000000007;
+        }
+        return (int)dp[s.length()];
+    }
+    
+    private int decodeChar(char c)
+    {
+        if(c=='*')
+            return 9;
+        else if(c=='0')
+            return 0;
+        
+        return 1;
+    }
+    
+    private int decodeTwoChars(char first, char second)
+    {
+        switch(first)
+        {
+            case '*':
+                if(second == '*')
+                    return 15;
+                else if(second>='0' && second<='6')
+                    return 2;
+                else
+                    return 1;
+            
+            case '1':
+                if(second == '*')
+                    return 9;
+                else
+                    return 1;
+            
+            case '2':
+                if(second == '*')
+                    return 6;
+                else if(second >= '0' && second <= '6')
+                    return 1;
+                else
+                    return 0;
+            default:
+                return 0;
+        }
+    }
+    
+    public static void main(String args[])
+    {
+        Scanner sc = new Scanner(System.in);
+        String str = sc.nextLine();
+        DecodeWaysII ob = new DecodeWaysII();
+        System.out.println(ob.numDecodings(str));
+    }
+}
+
+/*
+ * Example 1:
+
+   Input: s = "*"
+   Output: 9
+   Explanation: The encoded message can represent any of the encoded messages "1", "2", "3", "4", "5", "6", "7", "8", or "9".
+   Each of these can be decoded to the strings "A", "B", "C", "D", "E", "F", "G", "H", and "I" respectively.
+   Hence, there are a total of 9 ways to decode "*".
+   
+   Example 2:
+
+   Input: s = "1*"
+   Output: 18
+   Explanation: The encoded message can represent any of the encoded messages "11", "12", "13", "14", "15", "16", "17", "18", or "19".
+   Each of these encoded messages have 2 ways to be decoded (e.g. "11" can be decoded to "AA" or "K").
+   Hence, there are a total of 9 * 2 = 18 ways to decode "1*".
+   
+   Example 3:
+
+   Input: s = "2*"
+   Output: 15
+   Explanation: The encoded message can represent any of the encoded messages "21", "22", "23", "24", "25", "26", "27", "28", or "29".
+   "21", "22", "23", "24", "25", and "26" have 2 ways of being decoded, but "27", "28", and "29" only have 1 way.
+   Hence, there are a total of (6 * 2) + (3 * 1) = 12 + 3 = 15 ways to decode "2*".
+ */