Algorithms—144.Binary Tree Preorder Traversal

/**
* Definition for a binary tree node.
* 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> list=new ArrayList<Integer>();
return preorder(root,list);
}
public List<Integer> preorder(TreeNode root,List<Integer> list){
if (root==null) {
return list;
}
list.add(root.val);
list=preorder(root.left,list);
list=preorder(root.right, list);
return list;
}
}