commit ad1ff383ef356305b9d6341ee46370b2b8caab9a
Author: NaiJi <naijiworld@protonmail.com>
Date:   Tue Aug 31 12:42:13 2021 +0300

    4.1

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..5761abc
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+*.o
diff --git a/4.1/sum.cpp b/4.1/sum.cpp
new file mode 100644
index 0000000..c3e8d8f
--- /dev/null
+++ b/4.1/sum.cpp
@@ -0,0 +1,14 @@
+#include <iostream>
+
+int sum(int n)
+{
+    return (n != 0)
+         ?  n + sum(n - 1)
+         :  n;
+}
+
+int main()
+{
+    int n = 5;
+    std::cout << "For n = " << n << " sum is " << sum(n) << '\n';
+}
diff --git a/4.1/sum_tail.cpp b/4.1/sum_tail.cpp
new file mode 100644
index 0000000..0e27694
--- /dev/null
+++ b/4.1/sum_tail.cpp
@@ -0,0 +1,19 @@
+#include <iostream>
+
+int sumStep(int n, int s)
+{
+    return (n != 0)
+         ?  sumStep(n - 1, n + s)
+         :  s;
+}
+
+int sum(int n)
+{
+    return sumStep(n, 0);
+}
+
+int main()
+{
+    int n = 5;
+    std::cout << "For n = " << n << " sum is " << sum(n) << '\n';
+}