Several bug fixes and improvements. (#887)

* fix the bugs of C code.

* Add a header figure.

* Improve the definition of tree node height.

codes/c/chapter_computational_complexity/recursion.c

@@ -20,12 +20,12 @@ int recur(int n) {
 /* 使用迭代模拟递归 */
 int forLoopRecur(int n) {
     int stack[1000]; // 借助一个大数组来模拟栈
-    int top = 0;
+    int top = -1;    // 栈顶索引
     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[top++] = i;
+        stack[1 + top++] = i;
     }
     // 归：返回结果
     while (top >= 0) {
@@ -64,6 +64,9 @@ int main() {
     res = recur(n);
     printf("\n递归函数的求和结果 res = %d\n", res);
 
+    res = forLoopRecur(n);
+    printf("\n使用迭代模拟递归求和结果 res = %d\n", res);
+
     res = tailRecur(n, 0);
     printf("\n尾递归函数的求和结果 res = %d\n", res);