378. 将二叉查找树转换成双链表

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。

实现思路：

两个方法，一个是利用数组保存二叉树中序遍历的结果，然后转换成双向链表。

第二个方法：因为双向链表的prev正好对应树的left，next对应right。所以在中序遍历的过程中直接建立链表。

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        this.val = val
        this.left, this.right = None, None
Definition of Doubly-ListNode
class DoublyListNode(object):

    def __init__(self, val, next=None):
        self.val = val
        self.next = self.prev = next
"""


#method 1 利用数组存储树，然后转成链表。T(n),S(n)
class Solution:
    """
    @param: root: The root of tree
    @return: the head of doubly list node
    """
    def bstToDoublyList(self, root):
        # write your code here
        if root is None:
            return None
        self.result = []
        self.dfs(root)
        
        dummy = DoublyListNode(0)
        pre = dummy
        
        for res in self.result:
            tmp = DoublyListNode(res)
            tmp.prev = pre
            pre.next = tmp
            pre = tmp
        
        dummy.next.prev = None
        
        return dummy.next
  
    
    def dfs(self, root):
        if root is None:
            return
        
        self.dfs(root.left)
        self.result.append(root.val)
        self.dfs(root.right)

#mothed 2 在遍历树的同时，建立链表，T(n),S(h)，h是树的高度
class Solution:
    """
    @param: root: The root of tree
    @return: the head of doubly list node
    """
    def __init__(self):
        self.listHead = None
        self.listTail = None
    
    def bstToDoublyList(self, root):
        # write your code here
        if root is None:
            return None
        
        self.bstToDoublyList(root.left)
        
        if self.listHead == None:
            self.listHead = DoublyListNode(root.val)
            self.listTail = self.listHead
        else:
            proot = DoublyListNode(root.val)
            self.listTail.next = proot
            proot.prev = self.listTail
            self.listTail = proot
        
        self.bstToDoublyList(root.right)
        return self.listHead