README_CN.md

144. 二叉树的前序遍历

给定一个二叉树，返回它的前序 遍历。

示例:

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,2,3]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

题解 (Python)

1. 递归

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

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        return [root.val] + \
            self.preorderTraversal(root.left) + \
            self.preorderTraversal(root.right)

2. 迭代

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

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        vals = []
        nodes = [root]
        while nodes:
            node = nodes.pop()
            vals.append(node.val)
            if node.right:
                nodes.append(node.right)
            if node.left:
                nodes.append(node.left)
        return vals