Convert Binary Search Tree to Doubly Linked List

题目：

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

分析：

这道题目就强调一句话，就是传递参数的时候，void dfs()，里面一定是DoublyListNode* &curr。千万千万要引用！！！！

解法：

class Solution {
public:
    /**
     * @param root: The root of tree
     * @return: the head of doubly list node
     */
    void dfs(TreeNode* root, DoublyListNode* &curr) {
        if (root == NULL) {
            return;
        }

        dfs(root->left, curr);
        curr->next = new DoublyListNode(root->val);
        curr = curr->next;
        dfs(root->right, curr);

    } 

    DoublyListNode* bstToDoublyList(TreeNode* root) {
        // Write your code here
        if (root == NULL) {
            return NULL;
        }

        DoublyListNode dummy(0);
        DoublyListNode* curr = &dummy;

        dfs(root, curr);
        return dummy.next;
    }
};