Added factorisation technique (#604)

* Added factorisation technique

* Update and rename Math/hcf.txt to math/greatest_common_divisor.cpp

* Update greatest_common_divisor.cpp

math/greatest_common_divisor.cpp

@@ -0,0 +1,27 @@
+// C++ program to find GCD of two numbers
+#include <iostream>
+
+// Recursive function to return gcd of a and b
+int gcd(int a, int b) {
+    // Everything divides 0
+    if (a == 0)
+       return b;
+    if (b == 0)
+       return a;
+
+    // base case
+    if (a == b)
+        return a;
+
+    // a is greater
+    if (a > b)
+        return gcd(a-b, b);
+    return gcd(a, b-a);
+}
+
+// Driver program to test above function
+int main() {
+    int a = 98, b = 56;
+    std::cout << "GCD of " << a << " and " << b << " is " << gcd(a, b);
+    return 0;
+}