Leetcode617 Merge Two Binary Trees

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