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

        // 1.初始化一个ArrayList用于动态添加数据
        ArrayList<Integer> resultList = new ArrayList<>();

        // 2.调用前序遍历递归函数
        process(resultList, root);

        // 3.将ArrayList转换成int[]并返回
        return listToArray(resultList);
    }

    // 前序遍历递归函数
    public void process(ArrayList<Integer> resultList, TreeNode root) {
        if (root== null) {
            return;
        }
        resultList.add(root.val);
        process(resultList, root.left);
        process(resultList, root.right);
    }

    // ArrayList转int[]
    public int[] listToArray(ArrayList<Integer> resultList) {
        int[] resultArray = new int[resultList.size()];
        for (int i = 0; i < resultArray.length; i++) {
            resultArray[i] = resultList.get(i);
        }
        return resultArray;
    }
}