preorder.java

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

https://oj.leetcode.com/submissions/detail/7555379/
Binary Tree Preorder Traversal