LeetCode144-二叉树的前序遍历

原题链接：https://leetcode-cn.com/problems/binary-tree-preorder-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 preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        def preorder(root: TreeNode):
            if not root:
                return
            res.append(root.val)
            preorder(root.left)
            preorder(root.right)
        res = list()
        preorder(root)
        return res

 

 

方法二：采用栈作为中间存储

# 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 preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # 1、每次从栈中弹出一个节点，记作cur
        # 2、打印(处理)cur
        # 3、先右再左入栈
        # 4、周而复始
        res = list()
        if root:
            stack = deque()
            stack.append(root)
            while stack:
                head = stack.pop()
                res.append(head.val)
                if head.right:
                    stack.append(head.right)
                if head.left:
                    stack.append(head.left)
        return res