1,二叉树的前序遍历
题目网址:https://leetcode-cn.com/problems/binary-tree-preorder-traversal/
给定一个二叉树,返回它的 前序 遍历
我们的思路很简单,访问根节点,递归遍历左子树,递归遍历右子树
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
if (root == null){
return result; //空链表
}