feat: maximum-depth-of-binary-tree solution

Author: sm9171

maximum-depth-of-binary-tree/sm9171.java

@@ -0,0 +1,10 @@
+class Solution {
+    public int maxDepth(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        int leftDepth = maxDepth(root.left);
+        int rightDepth = maxDepth(root.right);
+        return Math.max(leftDepth, rightDepth) + 1;
+    }
+}