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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def flatten(self, root):
        """
        :type root: TreeNode
        :rtype: void Do not return anything, modify root in-place instead.
        """
        if root == None:
            return None
        res = []
        stack = [root]
        
        while len(stack) != 0:
            curNode = stack[-1]
            del stack[-1]
            res.append(curNode)
            if curNode.right:
                stack.append(curNode.right)
            if curNode.left:
                stack.append(curNode.left)
        
        # Note that in preorder, the last node is bound to be leaf node,
        # so you don't have to redirect its left and right.
        for i in range(len(res)-1):
            res[i].left = None
            res[i].right = res[i+1]