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) {
        // write code here
        List<Integer> res = new ArrayList<>();
        preTrav(res, root);
        int[] arr = new int[res.size()];
        for(int i = 0; i < arr.length; i++) {
            arr[i] = res.get(i);
        }
        return arr;
    }
    public void preTrav(List<Integer> res, TreeNode root) {
        if(root == null) {
            return;
        }
        res.add(root.val);
        preTrav(res, root.left);
        preTrav(res, root.right);
    }
}