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> aList = new ArrayList<>();
        sort( root,  aList);

        int[] array = new int[aList.size()];

        for (int i = 0; i < aList.size(); i++) {
            array[i] = aList.get(i);
        }

        return array;
    }

    private void sort(TreeNode root, List<Integer> aList) {
        if (root == null) {
            return;
        }

        aList.add(root.val);
        sort(root.left, aList);
        sort(root.right, aList);
    }
}