题目描述：

递归

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

写法：