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;
}
}

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