LeetCode144-Binary-Tree-Preorder-Traversal

**LeetCode 144 : Binary Tree Preorder Traversa**l

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].

来源： https://leetcode.com/problems/binary-tree-preorder-traversal/

问题描述：

本题其实就是将二叉树按照前序遍历将其值保存在List中。

我的Java代码：

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