Update README.md

leetcode145：二叉树的后序遍历

Author: sisterAn

README.md

@@ -74,6 +74,7 @@
 #### 树
 - [leetcode144：二叉树的前序遍历](https://github.com/sisterAn/JavaScript-Algorithms/issues/37)
 - [leetcode94：二叉树的中序遍历](https://github.com/sisterAn/JavaScript-Algorithms/issues/38)
+- [leetcode145：二叉树的后序遍历](https://github.com/sisterAn/JavaScript-Algorithms/issues/40)
 
 #### 编程题
 - [携程&蘑菇街&bilibili：手写数组去重、扁平化函数](https://github.com/sisterAn/JavaScript-Algorithms/issues/30)