. - 力扣（LeetCode）

/*
// 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 nullptr;dfs(root);head->left = pre;pre->right = head;return head;}
private:Node *pre, *head;void dfs(Node* cur) {if(cur == nullptr) return;dfs(cur->left);if(pre != nullptr) pre->right = cur;else head = cur;cur->left = pre;pre = cur;dfs(cur->right);}
};