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记忆点：

• 前序：VLR
• 中序：LVR
• 后序：LRV

举例：

一颗二叉树如下图所示：

则它的前序、中序、后序遍历流程如下图所示：

1.前序遍历

class Solution:
  def preorderTraversal(self, root: TreeNode):
 
    def preorder(root: TreeNode):
      if not root:
        return
      res.append(root.val)
      preorder(root.left)      
      preorder(root.right)
     
    res = []
    preorder(root)
    return res

2.中序遍历

class Solution:
  def inorderTraversal(self, root: TreeNode):
  
    def inorder(root: TreeNode):
      if not root:
        return
      inorder(root.left)
      res.append(root.val)
      inorder(root.right)
   
    res = []
    inorder(root)
    return res

3.后序遍历

class Solution:
  def postorderTraversal(self, root: TreeNode):
  
    def postorder(root: TreeNode):
      if not root:
        return
      postorder(root.left)
      res.append(root.val)
      postorder(root.right)
   
    res = []
    postorder(root)
    return res

4.测试

class TreeNode:
 def __init__(self, val=0, left=None, right=None):
  self.val = val
  self.left = left
  self.right = right

# 用列表递归创建二叉树
def createTree(root,list_n,i):
 if i <len(list_n):
  if list_n[i] == 'null':
    return None
  else:
   root = TreeNode(val = list_n[i])
   root.left = createTree(root.left,list_n,2*i+1)
   root.right = createTree(root.right,list_n,2*i+2)
   return root 
  return root
  
class Solution:
 def preorderTraversal(self, root: TreeNode): # 前序
 
  def preorder(root: TreeNode):
   if not root:
    return
   res.append(root.val)
   preorder(root.left)      
   preorder(root.right)
   
  res = []
  preorder(root)
  return res

 def inorderTraversal(self, root: TreeNode): # 中序
 
  def inorder(root: TreeNode):
   if not root:
    return
   inorder(root.left)
   res.append(root.val)
   inorder(root.right)
   
  res = []
  inorder(root)
  return res
  
 def postorderTraversal(self, root: TreeNode): # 后序
 
  def postorder(root: TreeNode):
   if not root:
    return
   postorder(root.left)
   postorder(root.right)
   res.append(root.val)
   
  res = []
  postorder(root)
  return res

if __name__ == "__main__":

 root = TreeNode()
 list_n = [1,2,3,4,5,6,7,8,'null',9,10]
 root = createTree(root,list_n,0)
 s = Solution()
 res_pre = s.preorderTraversal(root)
 res_in = s.inorderTraversal(root)
 res_post = s.postorderTraversal(root)
 print(res_pre)
 print(res_in)
 print(res_post)

5.结果

6.补充

6.1N叉树前序遍历

"""
# Definition for a Node.
class Node:
  def __init__(self, val=None, children=None):
    self.val = val
    self.children = children
"""

class Solution:
  def postorder(self, root: 'Node') -> List[int]:
    def seq(root):
      if not root:
        return
      res.append(root.val)
      for child in root.children:
        seq(child)      
    res = []
    seq(root)
    return res

N叉树后序遍历
"""
# Definition for a Node.
class Node:
  def __init__(self, val=None, children=None):
    self.val = val
    self.children = children
"""

class Solution:
  def postorder(self, root: 'Node') -> List[int]:
    def seq(root):
      if not root:
        return
      for child in root.children:
        seq(child)
      res.append(root.val)
    res = []
    seq(root)
    return res

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