不修改原来树的结构
public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
if (t1 == null) {
return t2;
}
if (t2 == null) {
return t1;
}
// 先合并根节点
TreeNode root = new TreeNode(t1.val + t2.val);
// 再递归合并左右子树
root.left = mergeTrees(t1.left, t2.left);
root.right = mergeTrees(t1.right, t2.right);
return root;
}