前序遍歷

# ------ coding:utf-8 -------
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def pre_order_recursive(root_node):
    if not root_node:
        return
    print root_node.val
    pre_order_recursive(root_node.left)
    pre_order_recursive(root_node.right)

def
 
 pre_order_non_recursive(root_node):
    if not root_node:
        return
    node_stack = []
    node = root_node
    while node_stack or node:
        # 從跟節點開始，把它的左子樹找出來
        while node:
            print node.val
            node_stack.append(node)
            node = node.left
        # 上面 while 的結束就是 node 為空的時候，也就是前一個節點沒有左子樹的了
 

        node = node_stack.pop()
        # 這個時候就開始檢視右子樹了
        node = node.right

中序遍歷

# ------ coding:utf-8 -------
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def mid_order_recursive(root_node):
    if not root_node:
        return
 

    mid_order_recursive(root_node.left)
    print root_node.val
    mid_order_recursive(root_node.right)

def mid_order_non_recursive(root_node):
    if not root_node:
        return
    node_stack = []
    node = root_node
    while node_stack or node:
        # 從跟節點開始，把它的左子樹找出來
        while node:
            node_stack.append(node)
            node = node.left
        # 上面 while 的結束就是 node 為空的時候，也就是前一個節點沒有左子樹的了
        node = node_stack.pop()
        print node.val
        # 這個時候就開始檢視右子樹了
        node = node.right

後序遍歷

# ------ coding:utf-8 -------
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def post_order_recursive(root_node):
    if not root_node:
        return
    post_order_recursive(root_node.left)
    post_order_recursive(root_node.right)
    print root_node.val


def post_order_non_recursive(root_node):
    if not root_node:
        return
    stack1 = [root_node]
    stack2 = []
    while stack1:
        # 在這個 while 迴圈裡面找到後序遍歷的逆序，存在在 stack2 中
        node = stack1.pop()
        if node.left:
            stack1.append(node.left)
        if node.right:
            stack1.append(node.right)
        stack2.append(node)
    while stack2:
        print stack2.pop().val

層次遍歷

# ------ coding:utf-8 -------
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def level_order(root_node):
    if not root_node:
        return
    node_queue = [root_node]
    while node_stack:
        # python的list中的pop函式，可以傳參，指定pop哪個位置的元素，預設最後一個
        node = node_queue.pop(0) 
        print node.val
        if node.left:
            node_queue.append(node.left)
        if node.right:
            node_queue.append(node.right)

Reference