Added Decode Ways CPP solution

Algorithms/Dynamic Programming/Decode Ways/readme.md

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+## Decode Ways
+
+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.
+
+**Example 1:**
+```
+Input: s = "12"
+
+Output: 2
+
+Explanation:
+
+"12" could be decoded as "AB" (1 2) or "L" (12).
+```
+
+**Example 2:**
+```
+Input: s = "226"
+
+Output: 3
+
+Explanation:
+
+"226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
+```
+
+**Example 3:**
+```
+Input: s = "06"
+
+Output: 0
+
+Explanation:
+
+"06" cannot be mapped to "F" because of the leading zero ("6" is different from "06"). In this case, the string is not a valid encoding, so return 0.
+```
+
+**Constraints:**
+- 1 <= s.length <= 100
+- s contains only digits and may contain leading zero(s).
+
+## Solution
+
+```cpp
+// C++ program to solve Decode Ways problem
+class Solution {
+public:
+    int numDecodings(string s) {
+        int n = s.size();
+
+        // If string begins with '0', no valid way to decode.
+        if(s[0] - '0' == 0){
+            return 0;
+        }
+        vector<int> dp(n+2 , 0);
+        // Initialize dp[0] as 1 -- only one way to decode a string with length of 1.
+        dp[0] =1;
+
+        // Loop through the length of the string.
+        for(int i = 0 ; i < n ; i++){
+            // If s[i] is nonzero, it can be included as a single digit.
+            if(s[i] != '0'){        
+                dp[i+1] += dp[i];
+            // If s[i] and s[i+1] form a valid two-digit number, add dp[i] to dp[i+2].
+            if( i<n-1 && ((s[i] -'0')*10 + s[i+1] - '0') <=26 ){
+              dp[i+2] += dp[i];
+            }
+        }
+        }
+        // Contains the number of ways to decode the string.
+        return dp[n];
+    }
+};
+```
+
+## Company Tags
+
+```
+Amazon
+Apple
+Bloomberg
+Commonvault
+Google
+Microsoft
+Salesforce
+TikTok
+```
+
+## Detailed Explanation
+
+[Click Here](https://prepfortech.io/leetcode-solutions/decode-ways)