二叉树的中序遍历

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        # 左 - 根 - 右
        if root is None:
            return []
        return self.inorderTraversal(root.left) + [root.val] + self.inorderTraversal(root.right)

class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        # 左 - 根 - 右
        if root is None: return []
        stack = []
        ans = []
        while root or stack:
            while root is not None:
                stack.append(root)
                root = root.left
            root = stack.pop()
            ans.append(root.val)
            root = root.right
        return ans