144. 二叉树的前序遍历

题目链接：https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
难度：简单

描述：
给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

提示：
节点数：[0, 100]

题解

# 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]:
        ret = []
        def recursion(root):
            if root is None:
                return
            ret.append(root.val)
            recursion(root.left)
            recursion(root.right)
        recursion(root)
        return ret

class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if root is None:
            return []
        ret = []
        stk = [root]
        while stk != []:
            temp_node = stk.pop()
            ret.append(temp_node.val)
            if temp_node.right is not None:
                stk.append(temp_node.right)
            if temp_node.left is not None:
                stk.append(temp_node.left)
        return ret