LeetCode94-二叉树的中序遍历

原题链接：https://leetcode-cn.com/problems/binary-tree-inorder-traversal/

方法一：递归，需了解树的递归序遍历

# 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]:
        def inorder(root: TreeNode):
            if not root:
                return
            inorder(root.left)
            res.append(root.val)
            inorder(root.right)
        res = list()
        inorder(root)
        return res

 

方法二：采用栈辅助

# 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
from collections import deque
class Solution:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # 每棵子树的整棵树左边界进栈，依次弹出的过程打印记录，对弹出节点的右树重复以上操作
        res = list()
        if root:
            stack = deque()
            while stack or root:
                if root:
                    # 左边界进栈
                    stack.append(root)
                    root = root.left
                else:
                    root = stack.pop()
                    res.append(root.val)
                    root = root.right
        return res