文章作者：Tyan
博客：noahsnail.com  |  CSDN  |  简书

1. 问题描述

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

2. 求解

这个题就是一个树的先序遍历问题，最简单的方案就是递归的遍历子树，要注意递归退出的条件。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<Integer>();
        if(root == null) {
            return list;
        }
        list.add(root.val);
        List<Integer> left = preorderTraversal(root.left);
        List<Integer> right = preorderTraversal(root.right);
        list.addAll(left);
        list.addAll(right);
        return list;
    }
}