将一个二叉查找树按照中序遍历转换成双向链表。

样例：
给定一个二叉查找树：

    4
   / \
  2   5
 / \
1   3

返回 1<->2<->3<->4<->5。

#ifndef C378_H
#define C378_H
#include<iostream>
#include<vector>
using namespace std;
class TreeNode{
public:
    int val;
    TreeNode *left, *right;
    TreeNode(int val)
    {
        this->val = val;
        this->left = this->right = NULL;
    }
};
class DoublyListNode{
public:
    int val;
    DoublyListNode *prev, *next;
    DoublyListNode(int val)
    {
        this->val = val;
        this->prev = this->next = NULL;
    }
};
class Solution {
public:
    /* * @param root: The root of tree * @return: the head of doubly list node */
    DoublyListNode * bstToDoublyList(TreeNode * root) {
        // write your code here
        if (root == NULL)
            return NULL;
        inOrder(root);
        int len = v.size();
        vector<DoublyListNode*> douList;
        for (auto c : v)
            douList.push_back(new DoublyListNode(c->val));
        if (len == 1)
        {
            douList[0]->next = NULL;
            douList[0]->prev = NULL;
            return douList[0];
        }
        for (int i = 0; i < len; ++i)
        {
            if (i == 0)
            {
                douList[i]->next = douList[i + 1];
                douList[i]->prev = NULL;
            }
            else if (i == len - 1)
            {
                douList[i]->next = NULL;
                douList[i]->prev = douList[i - 1];
            }
            else
            {
                douList[i]->next = douList[i + 1];
                douList[i]->prev = douList[i - 1];
            }
        }
        return douList[0];
    }
    void inOrder(TreeNode *root)
    {
        if (root == NULL)
            return;
        inOrder(root->left);
        v.push_back(root);
        inOrder(root->right);
    }
    vector<TreeNode*> v;
};
#endif