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 {
    /**
     * 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> res = new ArrayList<>();
        dfs(root, res);
        int[] list = new int[res.size()];
        for (int i = 0; i < res.size(); i++) {
            list[i] = res.get(i);
        }
        return list;
    }

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