leetcode: Binary Tree Preorder Traversal

非常基础的一道中序遍历二叉树的题目。递归的按照当前节点、左子结点、右子结点的顺序访问加入即可。

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public ArrayList<Integer> preorderTraversal(TreeNode root) {        ArrayList<Integer> res = new ArrayList<Integer>();        if( root == null )        {            return res;        }        res = fun(res,root);        return res;    }        ArrayList<Integer> fun(ArrayList<Integer> res,TreeNode root)    {        res.add(root.val);        if( root.left != null )        {            res = fun(res,root.left);        }                if( root.right != null )        {            res = fun(res,root.right);        }        return res;    }    }