【二叉树的中序遍历】

非递归：

# 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: TreeNode) -> List[int]:
        stack = []
        ans = []
        while root or stack:
            while root:
                stack.append(root)
                root = root.left
            if stack:
                root = stack.pop()
                ans.append(root.val)
                root = root.right
        return ans
        

递归：

# 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: TreeNode) -> List[int]:
        self.ans = []
        def func(root):
            if not root: return None
            func(root.left)
            self.ans.append(root.val)
            func(root.right)
        func(root)
        return self.ans