LeetCode by Java. binary-tree-preorder-traversal

原题描述：

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

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

分析：

先序遍历，遍历顺序为：根节点 -> 左子树 -> 右子树

代码：

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.ArrayList;public class Solution {    ArrayList<Integer> resList = new ArrayList<Integer>();    public ArrayList<Integer> preorderTraversal(TreeNode root) {       if(root == null) return resList; // 停止条件        resList.add(root.val); // 根        preorderTraversal(root.left);  // 左        preorderTraversal(root.right); // 右        return resList;    }}