第一次看到accept，心里有点小开心

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