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144. binary tree preorder traversal 解法

admin 11月 12, 2020 0

144. Binary Tree Preorder Traversal

Difficulty: Medium

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

Example:

Input: [1,null,2,3]
   1
    
     2
    /
   3

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

解法

¶法一

# 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]:
        stack = []
        res = []
        if not root:
            return []
        stack.append(root)
        while len(stack) != 0:
            node = stack.pop()
            res.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return res

¶法二

class :
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        ans, st = [],[]
        while root or st:
            while root:
                ans.append(root.val)
                st.append(root)
                root = root.left
            root = st[-1].right
            st.pop()

        return ans