feat: add python and java solutions to lcof problem

添加《剑指 Offer》题解：面试题55 - I. 二叉树的深度

Author: yanglbme

lcof/面试题55 - I. 二叉树的深度/README.md

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+# [面试题55 - I. 二叉树的深度](https://leetcode-cn.com/problems/er-cha-shu-de-shen-du-lcof/)
+
+## 题目描述
+输入一棵二叉树的根节点，求该树的深度。从根节点到叶节点依次经过的节点（含根、叶节点）形成树的一条路径，最长路径的长度为树的深度。
+
+例如：
+
+给定二叉树 `[3,9,20,null,null,15,7]`，
+
+```
+    3
+   / \
+  9  20
+    /  \
+   15   7
+```
+
+返回它的最大深度 3 。
+
+**提示：**
+
+- `节点总数 <= 10000`
+
+## 解法
+### Python3
+```python
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def maxDepth(self, root: TreeNode) -> int:
+        if root is None:
+            return 0
+        return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
+```
+
+### Java
+```java
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public int maxDepth(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
+    }
+}
+```
+
+### ...
+```
+
+```

lcof/面试题55 - I. 二叉树的深度/Solution.java

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+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public int maxDepth(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
+    }
+}

lcof/面试题55 - I. 二叉树的深度/Solution.py

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+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def maxDepth(self, root: TreeNode) -> int:
+        if root is None:
+            return 0
+        return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))