Description：
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.

题意：给定一颗二叉树s，判断二叉树t是否是s中某个节点的子树；

解法：我们只需要递归的判断二叉树t是否与二叉树s中某一个节点所在的子树相同即可；因此，我们首先需要实现的是如何判断两颗二叉树是相同的；之后，递归遍历所有的节点计算是否有一个节点所在的二叉树是否与t相同；

Java
/**
 * 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;
        return sametree(s, t) || isSubtree(s.left, t) || isSubtree(s.right, t);
    }
    private boolean sametree(TreeNode s, TreeNode t) {
        if (s == null && t == null) return true;
        if (s == null || t == null) return false;
        if (s.val != t.val) return false;
        return sametree(s.left, t.left) && sametree(s.right, t.right);
    }
}