LeetCode144：二叉树的前序遍历（递归+非递归）

目录

一、题目

二、示例

三、思路

四、代码

---

一、题目

给定一个二叉树，返回它的 前序遍历。

二、示例

示例:

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [1,2,3]

三、思路

1、递归。前序遍历：根-->左-->右

2、非递归，栈的思想，先进后出

四、代码

1、

# 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):
        if root is None:
            return []
        return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right)

if __name__ == '__main__':
    root = TreeNode(1)
    root.right = TreeNode(2)
    root.right.left = TreeNode(3)
    s = Solution()
    ans = s.preorderTraversal(root)
    print(ans)

2、

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# 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):
        res = []

        if root is None:
            return res
        # 先序遍历：根左右 栈：先进后出
        stack = [root]
        while(stack):
            root = stack.pop()
            res.append(root.val)
            if root.right:
                stack.append(root.right)
            if root.left:
                stack.append(root.left)
        return res

if __name__ == '__main__':
    root = TreeNode(1)
    root.right = TreeNode(2)
    root.right.left = TreeNode(3)
    s = Solution()
    ans = s.preorderTraversal(root)
    print(ans)