leetcode: 二叉树的三种非递归遍历

class TreeNode:
    def __init__(self,val):
        super(TreeNode).__init__()
        self.val = val
        self.left = None
        self.right = None
        self.isPop = False

def createTree(valList):
    if not valList:
        raise ValueError("input value list is None!")
    root = TreeNode(valList[0])
    valList.pop(0)
    layerOne = [root]
    layerTwo = []
    i = 0
    while i<len(valList):
        n = layerOne.pop(0)
        if valList[i] is not None:
            n.left = TreeNode(valList[i])
            layerOne.append(n.left)    
        i = i+1
        if i<len(valList) and valList[i] is not None:
            n.right = TreeNode(valList[i])
            layerOne.append(n.right)      
        i = i+1
    return root

valList = [1,2,3,4,None,6,7,8,9,None,11,12,13]
root = createTree(valList)

def inOrder(root):
    stack = []
    p = root
    while stack or p:
        if p:
            stack.append(p)
            p = p.left
        else:
            p = stack.pop()
            print(p.val,end = " ")
            p = p.right

    print()

def preOrder(root):
    if not root:
        return 
    stack = [root]
    while stack:
        p = stack.pop()
        print(p.val,end=" ")
        if p.right:
            stack.append(p.right)
        if p.left:
            stack.append(p.left)
    print()

def postOrder(root):
    def operate(node,stack):
        if node.left is None and node.right is None or node.isPop:
            print(node.val,end=" ")
        else:
            stack.append(node)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return stack

    if not root:
        return 
    stack = [root]
    while stack:
        p = stack.pop()    
        stack=operate(p,stack)
        p.isPop = True


preOrder(root)  #1 2 4 8 9 3 6 11 7 12 13
inOrder(root)   #8 4 9 2 1 6 11 3 12 7 13 
postOrder(root) #8 9 4 2 11 6 12 13 7 3 1

后序遍历的实现方法中，改变节点的入栈顺序也可以用来实现另外两种遍历

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