Add the section of iteration and recursion. (#693)

codes/cpp/chapter_computational_complexity/recursion.cpp

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+/**
+ * File: recursion.cpp
+ * Created Time: 2023-08-24
+ * Author: Krahets (krahets@163.com)
+ */
+
+#include "../utils/common.hpp"
+
+/* 递归 */
+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 */
+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 */
+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 */
+int main() {
+    int n = 5;
+    int res;
+
+    res = recur(n);
+    cout << "\n递归函数的求和结果 res = " << res << endl;
+
+    res = tailRecur(n, 0);
+    cout << "\n尾递归函数的求和结果 res = " << res << endl;
+
+    res = fib(n);
+    cout << "\n斐波那契数列的第 " << n << " 项为 " << res << endl;
+
+    return 0;
+}