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 void preOrder(List<Integer> list,TreeNode treeNode){
            if(treeNode==null){
                return ;
            }
            list.add((treeNode.val));
            preOrder(list,treeNode.left);
            preOrder(list,treeNode.right);
        }


    /**
     * 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
     */
    public int[] preorderTraversal (TreeNode root) {
        // write code here
    List<Integer> list=new ArrayList();
    preOrder(list,root);
    int arr[]=new int[list.size()];
    for(int i=0;i<list.size();i++){
        arr[i]=list.get(i);
    }
    return arr;
    
    }
}