二叉搜索树与双向链表

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;

    Node() {}

    Node(int _val) {
        val = _val;
        left = NULL;
        right = NULL;
    }

    Node(int _val, Node* _left, Node* _right) {
        val = _val;
        left = _left;
        right = _right;
    }
};
*/
class Solution {
public:
    Node* treeToDoublyList(Node* root) {
        if(root==nullptr)
            return root;
        if(root->left==nullptr&&root->right==nullptr)
        {
            root->left=root;
            root->right=root;
            return root;
        }
        
        stack<Node*> s;
        Node* pre=nullptr,*head=root;
        while(root||!s.empty())
        {
            if(root)
            {
                s.push(root);
                root=root->left;
            }
            else
            {
                root=s.top();
                s.pop();
                if(pre==nullptr)
                {
                    head=root;
                    pre=root;
                }
                else
                {
                    pre->right=root;
                    root->left=pre;
                    pre=root;
                }
                root=root->right;
            }
        }
        pre->right=head;
        head->left=pre;
        return head;
    }
};