feat: Update an image of binary expression tree (#568)

diff --git a/docs/basic/expression.md b/docs/basic/expression.md
index bce292a..9c7c9f8 100644 (file)
--- a/docs/basic/expression.md
+++ b/docs/basic/expression.md
@@ -6,7 +6,8 @@
 
 ## 递归
 
-递归的方法是把表达式拆分成表达式树（此处应有图），然后在树结构上自底向上进行运算。
+递归的方法是把表达式拆分成如图所示的表达式树，然后在树结构上自底向上进行运算。
+![](./images/bet.png)
 
 ## 非递归
 

diff --git a/docs/basic/images/bet.png b/docs/basic/images/bet.png
new file mode 100644 (file)
index 0000000..9b84b26
Binary files /dev/null and b/docs/basic/images/bet.png differ