二叉树问题---二叉树的前中后序的递归，非递归及Morris遍历

最新推荐文章于 2025-08-16 17:02:07 发布

wenbin1996

最新推荐文章于 2025-08-16 17:02:07 发布

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分类专栏：数据结构与算法文章标签： python 二叉树遍历

本文链接：https://blog.youkuaiyun.com/qq_34342154/article/details/76131445

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本文详细介绍了二叉树的基本概念及其多种遍历方法，包括递归与非递归实现方式，如前序、中序、后序遍历等，并提供了具体的Python代码实现。

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class TreeNode:
    def __init__(self, x):
        root.val = x
        root.left = None
        root.right = None

创建二叉树

def create_tree(root):
    element = input("Enter a key: ")
    if element == '#':
        root = None
    else:
        root = TreeNode(element)
        root.left = create_tree(root.left)
        root.right = create_tree(root.right)
    return root

前序遍历

def pre_order(root):
    if root:
        print(root.val,end=' ')
        pre_order(root.left)
        pre_order(root.right)

中序遍历

def mid_order(root):
    if root:
        mid_order(root.left)
        print(root.val, end=' ')
        mid_order(root.right)

后序遍历

def post_order(root):
    if root:
        post_order(root.left)
        post_order(root.right)
        print(root.val, end=' ')

非递归前序遍历

def preorder(root):
    if not root:
        return
    stack = []
    while root or len(stack):
        if root:
            stack.append(root)
            print(root.val, end=' ')
            root = root.left
        else:
            root = stack.pop()
            root = root.right

非递归中序遍历

def midorder(root):
    if not root:
        return
    stack = []
    while root or stack:
        if root:
            stack.append(root)
            root = root.left
        else:
            root = stack.pop()
            print(root.val, end=' ')
            root = root.right

非递归后序遍历

def postorder(root):
    if not root:
        return
    stack1 = []
    stack2 = []
    while root or stack1:
        if root:
            stack1.append(root)
            stack2.append(root.val)
            root = root.right
        else:
            root = stack1.pop()
            root = root.left
    while stack2:
        print(stack2.pop(), end=' ')

非递归后序遍历（使用一个栈）

说明：使用一个变量c来记录每次弹出的元素，保证元素不会重复进栈

def postOrder(root):
    if not root:
        return
    stack = []
    stack.append(root)
    c = None
    while stack:
        c = stack[-1]
        if c.left and c.left != root and c.right != root:
            stack.append(c.left)
        elif c.right and c.right != root:
            stack.append(c.right)
        else:
            print(stack.pop().val, end=' ')
            root = c

Morris前序遍历

def morrisPre(root):
    if not root:
        return
    while root:
        cur = root.left
        if cur:
            while cur.right and cur.right != root:
                cur = cur.right
            if not cur.right:
                cur.right = root
                print(root.val, end=' ')
                root = root.left
                continue
            else:
                cur.right = None
        else:
            print(root.val, end=' ')
        root = root.right

Morris中序遍历

def morrisIn(root):
    if not root:
        return
    while root:
        cur = root.left
        if cur:
            while cur.right and cur.right != root:
                cur = cur.right
            if not cur.right:
                cur.right = root
                root = root.left
                continue
            else:
                cur.right = None
        print(root.val, end=' ')
        root = root.right

Morris后序遍历

def morrisPost(root):
    def printRightEdge(root):
        tail = reverseEdge(root)
        cur = tail
        while cur:
            print(cur.val, end=' ')
            cur = cur.right
        reverseEdge(tail)

    def reverseEdge(root):
        pre = None
        while root:
            next = root.right
            root.right = pre
            pre = root
            root = next
        return pre

    if not root:
        return
    head = root
    while root:
        cur = root.left
        if cur:
            while cur.right and cur.right != root:
                cur = cur.right
            if not cur.right:
                cur.right = root
                root = root.left
                continue
            else:
                cur.right = None
                printRightEdge(root.left)
        root = root.right
    printRightEdge(head)

层次遍历

def levelorder(root):
    if not root:
        return
    queue = []
    queue.append(root)
    while queue:
        root = queue.pop(0)
        print(root.val, end=' ')
        if root.left:
            queue.append(root.left)
        if root.right:
            queue.append(root.right)