LintCode 378. 将二叉树转换成双链表（非递归遍历）

样例 1：
输入:
	    4
	   / \
	  2   5
	 / \
	1   3		

输出: 1<->2<->3<->4<->5

样例 2：
输入:
	    3
	   / \
	  4   1

输出:4<->3<->1

/**
  * Definition of Doubly-ListNode
 * class DoublyListNode {
 * public:
 *     int val;
 *     DoublyListNode *next, *prev;
 *     DoublyListNode(int val) {
 *         this->val = val;
 *         this->prev = this->next = NULL;
 *     }
 * } * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = 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) return NULL;
        stack<TreeNode*> stk;
        DoublyListNode *h = new DoublyListNode(-1), *pre = h;
        while(!stk.empty() || root)
        {
            while(root)
            {
                stk.push(root);
                root = root->left;
            }
            root = stk.top();
            stk.pop();
            DoublyListNode *cur = new DoublyListNode(root->val);
            pre->next = cur;
            cur->prev = pre;
            pre = cur;
            root = root->right;
        }
        DoublyListNode *head = h->next;
        h->next = NULL;
        head->prev = NULL;
        delete h;
        return head;
    }
};