feat(csharp): add the section of iteration and recursion (#697)

codes/csharp/chapter_computational_complexity/recursion.cs

@@ -0,0 +1,56 @@
+/**
+* File: recursion.cs
+* Created Time: 2023-08-28
+* Author: hpstory (hpstory1024@163.com)
+*/
+
+namespace hello_algo.chapter_computational_complexity;
+
+public class recursion {
+    /* 递归 */
+    public int recur(int n) {
+        // 终止条件
+        if (n == 1)
+            return 1;
+        // 递：递归调用
+        int res = recur(n - 1);
+        // 归：返回结果
+        return n + res;
+    }
+
+    /* 尾递归 */
+    public int tailRecur(int n, int res) {
+        // 终止条件
+        if (n == 0)
+            return res;
+        // 尾递归调用
+        return tailRecur(n - 1, res + n);
+    }
+
+    /* 斐波那契数列：递归 */
+    public int fib(int n) {
+        // 终止条件 f(1) = 0, f(2) = 1
+        if (n == 1 || n == 2)
+            return n - 1;
+        // 递归调用 f(n) = f(n-1) + f(n-2)
+        int res = fib(n - 1) + fib(n - 2);
+        // 返回结果 f(n)
+        return res;
+    }
+
+    /* Driver Code */
+    [Test]
+    public void Test() {
+        int n = 5;
+        int res;
+
+        res = recur(n);
+        Console.WriteLine("\n递归函数的求和结果 res = " + res);
+
+        res = tailRecur(n, 0);
+        Console.WriteLine("\n尾递归函数的求和结果 res = " + res);
+
+        res = fib(n);
+        Console.WriteLine("\n斐波那契数列的第 " + n + " 项为 " + res);
+    }
+}