牛客hot100--BM23---二叉树的前序遍历(简单难度)

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