二叉树先序遍历

描述

Given a binary tree, return the preorder traversal of its nodes' values.

思路

二叉树是典型的递归结构，使用递归可以轻易地做出解答。

解法二是使用栈，将未遍历的节点（子树）放入栈中。

解答

递归

from typing import List

# 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: TreeNode) -> List[int]:
        if not root:
            return []
        return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right)

栈

from typing import List

# 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: TreeNode) -> List[int]:
        if not root:
            return []
        result = []
        stack = [root]
        while stack:
            node = stack.pop()
            result.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return result

标签： Medium、二叉树、栈