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
        ArrayList<Integer> list = new ArrayList<>();
        preOrder(list, root);
        int[] nums = new int[list.size()];
        for(int i = 0; i < nums.length; i++){
            nums[i] = list.get(i);
        }

        return nums;
    }

    public void preOrder(ArrayList<Integer> list, TreeNode root){
        if(root == null)return;

        list.add(root.val);
        preOrder(list, root.left);
        preOrder(list, root.right);
    }
}