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

codes/cpp/chapter_computational_complexity/iteration.cpp

@@ -0,0 +1,76 @@
+/**
+ * File: iteration.cpp
+ * Created Time: 2023-08-24
+ * Author: Krahets (krahets@163.com)
+ */
+
+#include "../utils/common.hpp"
+
+/* for 循环 */
+int forLoop(int n) {
+    int res = 0;
+    // 循环求和 1, 2, ..., n-1, n
+    for (int i = 1; i <= n; ++i) {
+        res += i;
+    }
+    return res;
+}
+
+/* while 循环 */
+int whileLoop(int n) {
+    int res = 0;
+    int i = 1; // 初始化条件变量
+    // 循环求和 1, 2, ..., n-1, n
+    while (i <= n) {
+        res += i;
+        i += 1; // 更新条件变量
+    }
+    return res;
+}
+
+/* while 循环（两次更新） */
+int whileLoopII(int n) {
+    int res = 0;
+    int i = 1; // 初始化条件变量
+    // 循环求和 1, 2, 4, 5...
+    while (i <= n) {
+        res += i;
+        i += 1; // 更新条件变量
+        res += i;
+        i *= 2; // 更新条件变量
+    }
+    return res;
+}
+
+/* 双层 for 循环 */
+string nestedForLoop(int n) {
+    ostringstream res;
+    // 循环 i = 1, 2, ..., n-1, n
+    for (int i = 1; i <= n; ++i) {
+        // 循环 j = 1, 2, ..., n-1, n
+        for (int j = 1; j <= n; ++j) {
+            res << "(" << i << ", " << j << "), ";
+        }
+    }
+    return res.str();
+}
+
+/* Driver Code */
+int main() {
+    int n = 5;
+    int res;
+
+    res = forLoop(n);
+    cout << "\nfor 循环的求和结果 res = " << res << endl;
+
+    res = whileLoop(n);
+    cout << "\nwhile 循环的求和结果 res = " << res << endl;
+
+    res = whileLoopII(n);
+    cout << "\nwhile 循环（两次更新）求和结果 res = " << res << endl;
+
+    string resStr = nestedForLoop(n);
+    cout << "\n双层 for 循环的遍历结果 " << resStr << endl;
+
+    return 0;
+}