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
       * 递归实现 跟左右，list放置遍历到的数据
       * @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<>();
        ps(root, list);

        int [] r = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            r[i] = list.get(i);
        }

        return r;
    }

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