leetcode 94. 二叉树的中序遍历

class Solution:
    def inorderTraversal(self, root: TreeNode):
        if not root:
            return []

        stk = []
        res = []
        cur = root
        
        while stk or cur:
            while cur:
                stk.append(cur)
                cur = cur.left

            cur = stk.pop()
            res.append(cur.val)
            cur = cur.right

        return res