二叉树三种遍历的极简解法 with Python

众所周知，二叉树存在三种常见遍历方式，分别为前序、中序和后序，节点读取顺序分别为中左右、左中右和左右中。这里提供一种通用的极简解法，三种遍历方法的代码如下。

前序遍历

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        ans = []

        def dfs(node):
            if not node:
                return 
            ans.append(node.val)
            dfs(node.left)
            dfs(node.right)
        
        dfs(root)
        return ans

中序遍历

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        ans = []

        def dfs(node):
            if not node:
                return 
            dfs(node.left)
            ans.append(node.val)
            dfs(node.right)
        
        dfs(root)
        return ans

后序遍历

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        ans = []

        def dfs(node):
            if not node:
                return 
            dfs(node.left)
            dfs(node.right)
            ans.append(node.val)
        
        dfs(root)
        return ans