Node* head,Node* pre;
void dfs(Node* cur)
{
    if(cur == null) return;
    dfs(cur->left);
    /******走到最左节点******/
    if(pre == null) head = cur;
    else pre->right = cur;
    /******链接.更新******/
    cur->left = pre;
    pre = cur;
    dfs(cur->right);
}
​
Node* treeTodoubleList(Node* root)
{
    if(root == null) return null;
    dfs(root);
    head->left = pre;
    pre->right = head;
    return head;
}