Convert Binary Search Tree to Doubly Linked List

Convert a binary search tree to doubly linked list with in-order traversal.

Have you met this question in a real interview?

Yes

Example

Given a binary search tree:

    4
   /   2   5
 / 1   3

return 1<->2<->3<->4<->5

Runtime: 15ms

 1 /**
 2  * Definition of TreeNode:
 3  * class TreeNode {
 4  * public:
 5  *     int val;
 6  *     TreeNode *left, *right;
 7  *     TreeNode(int val) {
 8  *         this->val = val;
 9  *         this->left = this->right = NULL;
10  *     }
11  * }
12  * Definition of Doubly-ListNode
13  * class DoublyListNode {
14  * public:
15  *     int val;
16  *     DoublyListNode *next, *prev;
17  *     DoublyListNode(int val) {
18  *         this->val = val;
19            this->prev = this->next = NULL;
20  *     }
21  * }
22  */
23 class Solution {
24 public:
25     /**
26      * @param root: The root of tree
27      * @return: the head of doubly list node
28      */
29     DoublyListNode* bstToDoublyList(TreeNode* root) {
30         // Write your code here
31
32         // do inorder traversal first
33         vector<int> inorderStore;
34         inorder(root, inorderStore);
35
36         // use the values in vector to construct a double linked list
37         DoublyListNode* pre= new DoublyListNode(0);
38         return constructDoubleList(inorderStore, pre);
39     }
40 private:
41     void inorder(TreeNode* root, vector<int>& inorderStore) {
42         if(!root) return;
43         inorder(root->left, inorderStore);
44         inorderStore.push_back(root->val);
45         inorder(root->right, inorderStore);
46     }
47
48     DoublyListNode* constructDoubleList(vector<int>& inorderStore, DoublyListNode* pre) {
49         if(inorderStore.empty()) return NULL;
50
51         DoublyListNode* move = pre;
52         for (int i = 0; i < inorderStore.size(); i++) {
53             DoublyListNode* temp = new DoublyListNode(inorderStore[i]);
54             move->next = temp;
55             move = move->next;
56         }
57
58         DoublyListNode* head = pre->next;
59         head->prev = NULL;
60         return head;
61     }
62 };