合并二叉树（力扣题库）

   public TreeNode mergeTrees(TreeNode t1, TreeNode t2){
        TreeNode t=null;
        if (t1!=null&&t2!=null){
            t= new TreeNode(t1.val+t2.val);
            t=new TreeNode(t1.val+t2.val);
            t.left= addleft(t.left,t1.left,t2.left);
            t.right= addright(t.right,t1.right,t2.right);
        }else if (t1==null&&t2!=null){
            t=new TreeNode(t2.val);
            t.left= addleft(t.left,null,t2.left);
            t.right=addright(t.right,null,t2.right);
        }else if (t1!=null&&t2==null){
            t=new TreeNode(t1.val);
            t.left= addleft(t.left,t1.left,null);
            t.right= addright(t.right,t1.right,null);
        }
    return t;
    }
    public TreeNode addright(TreeNode t,TreeNode t1,TreeNode t2){

        if (t1!=null&&t2!=null){
            t=new TreeNode(t1.val+t2.val);
           t.left= addleft(t.left,t1.left,t2.left);
           t.right= addright(t.right,t1.right,t2.right);
        }
        if (t1==null&&t2!=null){
            t=new TreeNode(t2.val);
            t.left= addleft(t.left,null,t2.left);
            t.right=addright(t.right,null,t2.right);
        }
        if (t1!=null&&t2==null){
            t=new TreeNode(t1.val);
           t.left= addleft(t.left,t1.left,null);
           t.right= addright(t.right,t1.right,null);
        }
        return t;
    }

    public TreeNode addleft(TreeNode t,TreeNode t1,TreeNode t2){

        if (t1!=null&&t2!=null){
            t=new TreeNode(t1.val+t2.val);
           t.left= addleft(t.left,t1.left,t2.left);
           t.right= addright(t.right,t1.right,t2.right);
        }
        if (t1==null&&t2!=null){
            t=new TreeNode(t2.val);
           t.left= addleft(t.left,null,t2.left);
           t.right= addright(t.right,null,t2.right);
        }
       if (t1!=null&&t2==null){
           t=new TreeNode(t1.val);
           t.left= addleft(t.left,t1.left,null);
          t.right= addright(t.right,t1.right,null);
       }
        return t;
    }