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 {

   

    public int[] preorderTraversal (TreeNode root) {
	  //创建Integer集合
       List<Integer> list = new ArrayList<>();
	  //先序遍历函数
       preOrder(root,list);
       int[] array = list.stream().mapToInt(Integer::intValue).toArray();
       return array;
    }

    public void preOrder(TreeNode node,List<Integer> list) {
        if (node == null) {
            return ;
        }
	  //根
        list.add(node.val);
	  //左
        preOrder(node.left,list);
	  //右
        preOrder(node.right,list);
        return ;
    }
}