144. 二叉树的前序遍历

原题

https://leetcode.cn/problems/binary-tree-preorder-traversal/description/

思路

递归

复杂度

时间：O(n)
空间：O(n)

Python代码

# 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]:
        ans = []

        def preOrder(root):
            if not root:
                return
            ans.append(root.val)
            preOrder(root.left)
            preOrder(root.right)

        preOrder(root)
        return ans

Go代码

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func preorderTraversal(root *TreeNode) []int {
	var ans []int
	var preOrder func(*TreeNode)
	preOrder = func(root *TreeNode) {
		if root == nil {
			return
		}
		ans = append(ans, root.Val)
		preOrder(root.Left)
		preOrder(root.Right)
	}
	preOrder(root)
	return ans
}