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:

     3
    / 
   4   5
  / 
 1   2

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:

     3
    / 
   4   5
  / 
 1   2
    /
   0

Given tree t:

   4
  / 
 1   2

Return false.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    public boolean isSubtree(TreeNode s, TreeNode t) {
        if(s==null&&t==null) return true;
        if(s==null&&t!=null) return false;
        if(s!=null&&t==null) return false;
        return isSameTree(s,t)||isSubtree(s.left,t)||isSubtree(s.right,t);   
    }
    
    public boolean isSameTree(TreeNode a, TreeNode b){
        if(a==null&&b==null) return true;
        if(a==null&&b!=null) return false;
        if(a!=null&&b==null) return false;
        if(a.val!=b.val) return false;
        return isSameTree(a.left,b.left)&&isSameTree(a.right,b.right);
    }
        
}