如果弄清楚了子樹，那么相同的樹比子樹的情況要簡單，碰到不相等可以直接返回。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSameTree(TreeNode p, TreeNode q) {
       return isSame(p,q);
    }
    private boolean isSame(TreeNode p, TreeNode q)
    {
         if(p==null||q==null)
        {
            if(p==null&&q==null)
                return true;
            else
                return false;
        }
        if(p.val==q.val)
            return isSame(p.left,q.left)&&isSame(p.right,q.right);
        else
            return false;
    }
}