94. 二叉树的中序遍历

题目链接：https://leetcode-cn.com/problems/binary-tree-inorder-traversal/
难度：简单

描述：
给定一个二叉树的根节点 root ，返回 它的 中序 遍历 。

题解

# 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]:
        ret = []
        def recursion(root):
            if root is None:
                return
            recursion(root.left)
            ret.append(root.val)
            recursion(root.right)
        recursion(root)
        return ret

# 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]:
        ret = []
        stk = []
        while root is not None or stk != []:
            while root is not None:
                stk.append(root)
                root = root.left
                root = stk.pop()
                ret.append(root.val)
                root = root.right
        return ret