feature: decode way dp sol added

Author: Debashis08

include/0005_DynamicProgramming/0007_DecodeWays.h

@@ -27,5 +27,6 @@ namespace DecodeWays
 		int CountWaysRecursiveHelper(string& digits, size_t index);
 	public:
 		int RecursiveCountWays(string digits);
+		int DpCountways(string digits);
 	};
 }

source/0005_DynamicProgramming/0007_DecodeWays.cc

@@ -32,4 +32,30 @@ namespace DecodeWays
 	{
 		return this->CountWaysRecursiveHelper(digits, 0);
 	}
+
+	int DynamicProgramming::DpCountways(string digits)
+	{
+		size_t digitsLength = digits.size();
+
+		vector<int> dp(digitsLength + 1, 0);
+
+		dp[digitsLength] = 1;
+
+		for (int index = digitsLength - 1; index >= 0; index--)
+		{
+			// Single digit decoding: check if current digit is not '0'.
+			if (digits[index] != '0')
+			{
+				dp[index] = dp[index + 1];
+			}
+
+			// Two digit decoding: check if next two digits are valid.
+			if ((index + 1 < digitsLength) && ((digits[index] == '1' && digits[index + 1] <= '9') || (digits[index] == '2' && digits[index + 1] <= '6')))
+			{
+				dp[index] += dp[index + 2];
+			}
+		}
+
+		return dp[0];
+	}
 }

test/0005_DynamicProgramming/0007_DecodeWaysTest.cc

@@ -16,4 +16,18 @@ namespace DecodeWays
 		// Assert
 		ASSERT_EQ(expectedWaysCount, actualWaysCount);
 	}
+
+	TEST(DecodeWays, DpTest01)
+	{
+		// Arrange
+		DynamicProgramming dp;
+		string digits = "121";
+		int expectedWaysCount = 3;
+
+		// Act
+		int actualWaysCount = dp.DpCountways(digits);
+
+		// Assert
+		ASSERT_EQ(expectedWaysCount, actualWaysCount);
+	}
 }