六.二叉树遍历

1. 二叉树前序遍历

1.1 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 preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        def dfs(root: TreeNode):
            if root is None: return
            res.append(root.val)
            dfs(root.left)
            dfs(root.right)
        res = []
        dfs(root)
        return res

1.2 迭代

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val