feat(ml):$104.maximum-depth-of-binary-tree.md (#450)

添加Java语言支持

Author: ztianming

problems/104.maximum-depth-of-binary-tree.md

@@ -64,7 +64,7 @@ var maxDepth = function (root) {
 
 ## 代码
 
-- 语言支持：JS，C++，Python
+- 语言支持：JS，C++，Java，Python
 
 JavaScript Code:
 
@@ -149,6 +149,55 @@ public:
 };
 ```
 
+Java Code:
+
+```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;
+        }
+        // 队列
+        Queue<TreeNode> queue = new LinkedList<TreeNode>();
+        queue.offer(root);
+        int res = 0;
+        // 按层扩展
+        while(!queue.isEmpty())
+        {
+            // 拿出该层所有节点，并压入子节点
+            int size = queue.size();
+            while(size > 0)
+            {
+                TreeNode node = queue.poll();
+
+                if(node.left != null)
+                {
+                    queue.offer(node.left);
+                }
+                if(node.right != null)
+                {
+                    queue.offer(node.right);
+                }
+                size-=1;
+            }
+            // 统计层数
+            res +=1;
+        }
+        return res;
+    }
+}
+```
+
 Python Code:
 
 ```python