Subtree of Another Tree

Given two non-empty binary trees s and t, check whether tree t has exactly the same structure and node values with a subtree of s. A subtree of s is a tree consists of a node in s and all of this node's descendants. The tree s could also be considered as a subtree of itself.

Example 1:
Given tree s:

Given tree t:

   4 
  / 
 1   2

Return true, because t has the same structure and node values with a subtree of s.

Example 2:
Given tree s:

Given tree t:

   4
  / 
 1   2

Return false.

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public boolean isSubtree(TreeNode s, TreeNode t) {
12         if (s == null) return false;
13         
14         if (isSame(s, t)) return true;
15         return isSubtree(s.left, t) || isSubtree(s.right, t);
16     }
17     
18     private boolean isSame(TreeNode s, TreeNode t) {
19         if (s == null && t == null) return true;
20         if (s == null || t == null) return false;
21         
22         return s.val == t.val &&
23                isSame(s.left, t.left) &&
24                isSame(s.right, t.right);
25     }
26 }