二叉树的前序、中序、后序遍历非递归形式Python实现

二叉树的前序、中序、后序遍历需要用到数据结构栈。

下面给出一种统一写法：

下面是中序遍历的写法：

class Solution2:
    def midOrder(self, root):
        stack = [root]
        res = []
        while stack:
            s = stack.pop()
            if type(s) is Node:
                if s.right:
                    stack.append(s.right)
                stack.append(s.val)
                if s.left:
                    stack.append(s.left)
            else:
                res.append(s)
        return res

如果改成前序遍历：（只需要将入栈的顺序改一下就可以了）

def midOrder(self, root):
    stack = [root]
    res = []
    while stack:
        s = stack.pop()
        if type(s) is Node:
            if s.right:
                stack.append(s.right)
            if s.left:
                stack.append(s.left)
            stack.append(s.val)
        else:
            res.append(s)
    return res

下面是后续：

def midOrder(self, root):
    stack = [root]
    res = []
    while stack:
        s = stack.pop()
        if type(s) is Node:
            stack.append(s.val)
            if s.right:
                stack.append(s.right)
            if s.left:
                stack.append(s.left)
        else:
            res.append(s)
    return res