合并二叉树-递归

非常简单的代码^_^
解题思路：

1. 两个节点的值求和并赋给t1
2. 递归合并t1、t2的左右节点。

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