LeetCode144：Binary Tree Preorder Traversal

最新推荐文章于 2024-05-30 07:40:02 发布

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本文介绍了一种常见的二叉树遍历方法——前序遍历，包括递归和非递归两种实现方式。递归实现简洁明了，而非递归实现则使用栈结构进行节点存储和遍历。

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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?

LeetCode：链接

二叉树的前序遍历。

1）递归实现：

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

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res = []
        def dfs(root, res):
            if not root:
                return []
            res.append(root.val)
            dfs(root.left, res)
            dfs(root.right, res)
            return res
        dfs(root, res)
        return res

2）非递归实现：

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

class Solution(object):
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        stack = []
        res = []
        while root or stack:
            if root:
                res.append(root.val)
                if root.right:
                    stack.append(root.right)
                root = root.left
            else:
                root = stack.pop()
        return res