二叉树（1）-----遍历

一、前序遍历：

递归方式：

def preorder(tree):
    if tree:
        print(tree.val)
        preorder(tree.getLeftChild())
        preorder(tree.getRightChild())

非递归方式：时间复杂度O（n)，空间复杂度O（n）

def preOrder(head):
    if not head:
        return None
    res ,stack = [] , [head]
    while stack:
        cur = stack.pop()
        res.append(cur.val)
        if cur.right:
            stack.append(cur.right)
        if cur.left:
            stack.append(cur.left)
    return res

二、中序遍历：

递归方式：

def inorder(tree):    
    if tree:
        inorder(tree.left)
        print(tree.val)
        inorder(tree.right)

非递归方式：

def InOrder(head):
    if not head:
        return None
    res ,stack = [] , []
    cur = head
    while stack or cur:
        if cur:
            stack.append(cur)
            cur = cur.left
        else:
            cur = stack.pop()
            res.append(cur.val)
            cur =cur.right
    return res

 三、后序遍历：

递归

def postorder(tree):
    if tree:
        postorder(tree.left)
        postorder(tree.right))
        print(tree.val)

非递归：

def PosOrder(head):
    if not head:
        return []
    s1 , s2 = [head] , []
    cur = head
    while s1:
        cur = s1.pop()
        s2.append(cur.val)
        if cur.left:
            s1.append(cur.left)
        if cur.right:
            s1.append(cur.right)
    return s2[::-1]