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) return false;
        if(t == null) return true;
        if(isSameTree(s, t)) return true;
        return 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.val != b.val) return false;
        return isSameTree(a.left, b.left) && isSameTree(a.right, b.right);
    }
}