简单直接的版本：把两个树合并到t1。若两个树都存在，直接把和放在t1中，然后再合并子树。合并子树之前进行一下t1的空节点判断即可

import java.util.*;
public class Solution {
    public TreeNode mergeTrees (TreeNode t1, TreeNode t2) {
         if(t1!=null&&t2!=null){
             t1.val = t1.val+t2.val;
             if(t1.left!=null&&t2.left!=null)
                 mergeTrees(t1.left,t2.left);
             if(t1.right!=null&&t2.right!=null)
                 mergeTrees(t1.right,t2.right);
             if(t1.left==null)
                 t1.left = t2.left;
             if(t1.right==null)
                 t1.right = t2.right;
         }
        return t1;
    }
}