Update Readme.md

Author: wisdompeak

Readme.md

@@ -256,6 +256,7 @@
 [1522.Diameter-of-N-Ary-Tree](https://github.com/wisdompeak/LeetCode/tree/master/Tree/1522.Diameter-of-N-Ary-Tree) (M) 
 [2049.Count-Nodes-With-the-Highest-Score](https://github.com/wisdompeak/LeetCode/tree/master/Tree/2049.Count-Nodes-With-the-Highest-Score) (M+) 
 [2246.Longest-Path-With-Different-Adjacent-Characters](https://github.com/wisdompeak/LeetCode/tree/master/Tree/2246.Longest-Path-With-Different-Adjacent-Characters) (M+) 
+[2538.Difference-Between-Maximum-and-Minimum-Price-Sum](https://github.com/wisdompeak/LeetCode/tree/master/Tree/2538.Difference-Between-Maximum-and-Minimum-Price-Sum) (H) 
 * ``Serialization & Hashing`` 
 [297.Serialize-and-Deserialize-Binary-Tree](https://github.com/wisdompeak/LeetCode/tree/master/Tree/297.Serialize-and-Deserialize-Binary-Tree) （H-） 
 [652.Find-Duplicate-Subtrees](https://github.com/wisdompeak/LeetCode/tree/master/Tree/652.Find-Duplicate-Subtrees) (H)