Update README.md

Author: jasoncao11

README.md

@@ -93,6 +93,10 @@ LeetCode 精选算法题, Python题解+详细注释，持续更新。
 - [37. 将有序数组转换为二叉搜索树](https://github.com/jasoncao11/Algorithm-notebook/blob/master/37.%20%E5%B0%86%E6%9C%89%E5%BA%8F%E6%95%B0%E7%BB%84%E8%BD%AC%E6%8D%A2%E4%B8%BA%E4%BA%8C%E5%8F%89%E6%90%9C%E7%B4%A2%E6%A0%91.md)
 - [41. 反转链表](https://github.com/jasoncao11/Algorithm-notebook/blob/master/41.%20%E5%8F%8D%E8%BD%AC%E9%93%BE%E8%A1%A8.md)
 - [46. 二叉树中的最大路径和](https://github.com/jasoncao11/Algorithm-notebook/blob/master/46.%20%E4%BA%8C%E5%8F%89%E6%A0%91%E4%B8%AD%E7%9A%84%E6%9C%80%E5%A4%A7%E8%B7%AF%E5%BE%84%E5%92%8C.md)
+- [77. 平衡二叉树](https://github.com/jasoncao11/Algorithm-notebook/blob/master/77.%20%E5%B9%B3%E8%A1%A1%E4%BA%8C%E5%8F%89%E6%A0%91.md)
+- [78. 二叉树展开为链表](https://github.com/jasoncao11/Algorithm-notebook/blob/master/78.%20%E4%BA%8C%E5%8F%89%E6%A0%91%E5%B1%95%E5%BC%80%E4%B8%BA%E9%93%BE%E8%A1%A8.md)
+- [79. 合并二叉树](https://github.com/jasoncao11/Algorithm-notebook/blob/master/79.%20%E5%90%88%E5%B9%B6%E4%BA%8C%E5%8F%89%E6%A0%91.md)
+- [80. 翻转二叉树](https://github.com/jasoncao11/Algorithm-notebook/blob/master/80.%20%E7%BF%BB%E8%BD%AC%E4%BA%8C%E5%8F%89%E6%A0%91.md)
 
 ### 8. 二分法