feat(二叉树理论基础.md): 新增java版本

problems/二叉树理论基础.md

@@ -188,6 +188,21 @@ struct TreeNode {
 
 Java：
 
+```java
+public class TreeNode {
+    int val;
+  	TreeNode left;
+  	TreeNode right;
+  	TreeNode() {}
+  	TreeNode(int val) { this.val = val; }
+  	TreeNode(int val, TreeNode left, TreeNode right) {
+    		this.val = val;
+    		this.left = left;
+    		this.right = right;
+  	}
+}
+```
+
 
 Python：