题解 | #二叉树的前序遍历#

import java.util.*;
import java.util.ArrayList;
import java.util.List;

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

    }

    public void preorder(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        list.add(root.val);
        preorder(root.left, list);
        preorder(root.right, list);
    }
}