递归
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 */ public static void preorder(List<Integer> list,TreeNode root){ if(root==null){ return ; } list.add(root.val); preorder(list,root.left); preorder(list,root.right); } public int[] preorderTraversal (TreeNode root) { // write code here List<Integer> list=new ArrayList<>(); preorder(list,root); int n=list.size(); int[] result=new int[n]; for(int i=0;i<n;i++){ result[i]=list.get(i); } return result; } }