Add the initial EN translation for C++ code (#1346)

en/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"
+
+/* Recursion */
+int recur(int n) {
+    // Termination condition
+    if (n == 1)
+        return 1;
+    // Recursive: recursive call
+    int res = recur(n - 1);
+    // Return: return result
+    return n + res;
+}
+
+/* Simulate recursion with iteration */
+int forLoopRecur(int n) {
+    // Use an explicit stack to simulate the system call stack
+    stack<int> stack;
+    int res = 0;
+    // Recursive: recursive call
+    for (int i = n; i > 0; i--) {
+        // Simulate "recursive" by "pushing onto the stack"
+        stack.push(i);
+    }
+    // Return: return result
+    while (!stack.empty()) {
+        // Simulate "return" by "popping from the stack"
+        res += stack.top();
+        stack.pop();
+    }
+    // res = 1+2+3+...+n
+    return res;
+}
+
+/* Tail recursion */
+int tailRecur(int n, int res) {
+    // Termination condition
+    if (n == 0)
+        return res;
+    // Tail recursive call
+    return tailRecur(n - 1, res + n);
+}
+
+/* Fibonacci sequence: Recursion */
+int fib(int n) {
+    // Termination condition f(1) = 0, f(2) = 1
+    if (n == 1 || n == 2)
+        return n - 1;
+    // Recursive call f(n) = f(n-1) + f(n-2)
+    int res = fib(n - 1) + fib(n - 2);
+    // Return result f(n)
+    return res;
+}
+
+/* Driver Code */
+int main() {
+    int n = 5;
+    int res;
+
+    res = recur(n);
+    cout << "\nSum result of the recursive function res = " << res << endl;
+
+    res = forLoopRecur(n);
+    cout << "\nSum result using iteration to simulate recursion res = " << res << endl;
+
+    res = tailRecur(n, 0);
+    cout << "\nSum result of the tail-recursive function res = " << res << endl;
+
+    res = fib(n);
+    cout << "The " << n << "th number in the Fibonacci sequence is " << res << endl;
+
+    return 0;
+}