Add comparison between iteration and recursion.

Fix the figure of tail recursion.
Fix two links.

codes/cpp/chapter_computational_complexity/recursion.cpp

@@ -17,6 +17,26 @@ int recur(int n) {
     return n + res;
 }
 
+/* 使用迭代模拟递归 */
+int forLoopRecur(int n) {
+    // 使用一个显式的栈来模拟系统调用栈
+    stack<int> stack;
+    int res = 0;
+    // 递：递归调用
+    for (int i = n; i > 0; i--) {
+        // 通过“入栈操作”模拟“递”
+        stack.push(i);
+    }
+    // 归：返回结果
+    while (!stack.empty()) {
+        // 通过“出栈操作”模拟“归”
+        res += stack.top();
+        stack.pop();
+    }
+    // res = 1+2+3+...+n
+    return res;
+}
+
 /* 尾递归 */
 int tailRecur(int n, int res) {
     // 终止条件
@@ -45,6 +65,9 @@ int main() {
     res = recur(n);
     cout << "\n递归函数的求和结果 res = " << res << endl;
 
+    res = forLoopRecur(n);
+    cout << "\n使用迭代模拟递归求和结果 res = " << res << endl;
+
     res = tailRecur(n, 0);
     cout << "\n尾递归函数的求和结果 res = " << res << endl;