Update 二叉树相关.md

blogs/Algorithm/剑指 Offer/二叉树相关.md

@@ -194,3 +194,45 @@ class Solution {
 }
 ```
 
+#### [55 - I. 二叉树的深度](https://leetcode-cn.com/problems/er-cha-shu-de-shen-du-lcof/)
+
+```java
+class Solution {
+
+    public int maxDepth(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
+    }
+}
+```
+
+```java
+class Solution {
+
+    public int maxDepth(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        int result = 0;
+        Deque<TreeNode> queue = new LinkedList<>();
+        queue.add(root);
+        while (!queue.isEmpty()) {
+            result++;
+            int n = queue.size();
+            for (int i = 0; i < n; i++) {
+                TreeNode node = queue.poll();
+                if (node.left != null) {
+                    queue.add(node.left);
+                }
+                if (node.right != null) {
+                    queue.add(node.right);
+                }
+            }
+        }
+        return result;
+    }
+}
+```
+