二叉树的遍历【递归、非递归】以及自己的感受

--------又来了一次二叉树的遍历，每次都有不一样的理解！真不知道那些大佬是如何想出这些牛逼的算法的，佩服至极！！！！大家多写，多想，对这些的理解真的会发生变化！！！
～～～～～～～～
--------这里多说一句哈，就是大家一定要多写多想！有很多知识是学的时候怎么都理解不了，过段时间回来在看，直接就明白了，连自己都懵了！融会贯通不就如此嘛！！！！我的学习方法就是四个字：反反复复！！！！

微信图片_20191224172703.jpg

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


# 先序  【递归】
def pre_order(root):
    if not root:
        return None
    print(root.val)
    pre_order(root.left)
    pre_order(root.right)


# 中序  【递归】
def mid_order(root):
    if not root:
        return None
    mid_order(root.left)
    print(root.val)
    mid_order(root.right)


# 后序  【递归】
def last_order(root):
    if not root:
        return None
    last_order(root.left)
    last_order(root.right)
    print(root.val)


# 先序  【循环】
def pre(root):
    if not root:
        return None
    stack = []
    tmp = root
    while stack or tmp:
        while tmp:
            print(tmp.val)
            stack.append(tmp)
            tmp = tmp.left
        node = stack.pop()
        tmp = node.right


# 中序  【循环】
def mid(root):
    if not root:
        return None
    stack = []
    tmp = root
    while stack or tmp:
        while tmp:
            stack.append(tmp)
            tmp = tmp.left
        node = stack.pop()
        print(node.val)
        tmp = node.right


# 后序  【循环】
def last(root):
    if not root:
        return None
    stack = []
    tmp = root
    while stack or tmp:
        while tmp:
            stack.append(tmp)
            tmp = tmp.left
        node = stack[-1]
        tmp = node.right
        if tmp is None:
            node = stack.pop()
            print(node.val)
            while stack and node == stack[-1].right:
                node = stack.pop()
                print(node.val)


if __name__ == '__main__':
    node1 = TreeNode(1)
    node2 = TreeNode(2)
    node3 = TreeNode(3)
    node4 = TreeNode(4)
    node5 = TreeNode(5)
    node6 = TreeNode(6)
    node7 = TreeNode(7)

    node1.left = node2
    node1.right = node3
    node2.left = node4
    node2.right = node5
    node3.left = node6
    node3.right = node7

    # pre_order(node1)
    # mid_order(node1)
    # last_order(node1)
    # pre(node1)
    # mid(node1)
    last(node1)