chore(CPlusPlus): adding recursion programs (#402)

Co-authored-by: Arsenic <54987647+Arsenic-ATG@users.noreply.github.com>

algorithms/CPlusPlus/README.md

@@ -119,3 +119,7 @@
 1. [Tower of Hanoi](Recursion/towerofHanoi.cpp)
 2. [Factorial](Recursion/factorial.cpp)
 3. [Permutation](Recursion/permutation.cpp)
+4. [GCD/HCF of two numbers](Recursion/GCD-of-two-numbers.cpp)
+5. [Sum of all elements of an array](Recursion/Sum-of-all-elements-in-an-array.cpp)
+6. [Decimal number to Binary conversion](Recursion/decimal-to-binary-conversion.cpp)
+7. [Sum of digits of a decimal integer](Recursion/sum-of-digits-of-an-integer.cpp)

algorithms/CPlusPlus/Recursion/GCD-of-two-numbers.cpp

@@ -0,0 +1,20 @@
+// Program to find GCD of two numbers using recursion
+// Executable link: https://ide.geeksforgeeks.org/wixyiKNFMi
+
+#include <iostream>
+
+using namespace std;
+
+// Also known as Euclidean algorithm
+// There exists an inbuilt function for gcd also __gcd(T a, T b) with Template T
+int euclidean_gcd(int a, int b) {
+    if (b == 0)
+        return a;
+    return euclidean_gcd(b, a%b);
+}
+
+int main() {
+    int n1 = 56, n2 = 65;
+    cout << "GCD of " << n1 << " and " << n2 << " is " << euclidean_gcd(n1, n2); 
+	return 0;
+}

algorithms/CPlusPlus/Recursion/Sum-of-all-elements-in-an-array.cpp

@@ -0,0 +1,18 @@
+// Program to find sum of all integer elements in an array
+// Executable link: https://ide.geeksforgeeks.org/cey3HeTjYK
+
+#include <iostream>
+
+using namespace std;
+
+int findSum(int A[], int size) {
+    if (n <= 0)
+        return 0;
+    return (findSum(A, size-1) + A[size-1]);
+}
+
+int main() {
+    int A[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+    cout << "The sum of the array is " << findSum(A, 10);
+    return 0;
+}

algorithms/CPlusPlus/Recursion/decimal-to-binary-conversion.cpp

@@ -0,0 +1,26 @@
+// Program to convert Decimal to Binary
+// Negative numbers are expressed here as Signed magnitude representation
+// Executable link: https://ide.geeksforgeeks.org/Cubhhp1jIl
+
+#include <iostream>
+
+using namespace std;
+
+// Trap: Remember the limit of int in C++
+// if a binary of a number exceed the limit of int, it will overflow
+// Tip: Use long long (and unsigned long long) instead
+unsigned long long decimalToBinary (int decimal_number) {
+    if (decimal_number == 0)
+        return 0;
+    else
+        return (decimal_number%2 + 10*decimalToBinary(decimal_number/2));
+}
+
+int main() {
+    int numb = 5;
+    int signed_bit = 0;
+    if(numb < 0)
+        signed_bit = 1;
+    cout << "Binary of " << numb << " is " << signed_bit << decimalToBinary(numb);
+    return 0;
+}

algorithms/CPlusPlus/Recursion/sum-of-digits-of-an-integer.cpp

@@ -0,0 +1,19 @@
+// Program to find sum of digits of an integer (Decimal Integer)
+// Executable link: https://ide.geeksforgeeks.org/pHHmcxVNKZ
+
+#include <iostream>
+
+using namespace std;
+
+// Replace 10 by 2 for binary numbers, 8 for octal numbers
+int sum_of_digits(int n) {
+    if (n == 0)
+        return 0;
+    return (n%10 + sum_of_digits(n/10));
+}
+
+int main() {
+    int numb = 12345;
+    cout << "Sum of digits of " << numb << " is " << sum_of_digits(numb);
+    return 0;
+}