题解 | #二叉树的前序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型一维数组
    List<Integer> list = new ArrayList<>();
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        int[] array = new int[list.size()];
        int k = 0;
        for(int num : list){
            array[k++] = num;
        }
        return array;
    public void preorder(TreeNode root){
        if(root==null){
            return;
        }
        list.add(root.val);
        preorder(root.left);
        preorder(root.right);
    }
}