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) {
        List<Integer> list=new ArrayList<>();
        dfs(list,root);
        int[] res=new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            res[i]=list.get(i);
        }
        return res;
    }
    public void dfs(List<Integer> list,TreeNode root){
        if(root!=null){
            list.add(root.val);
            dfs(list,root.left);
            dfs(list,root.right);
        }
    }
}