LeetCode_144Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:
Given binary tree {1,#,2,3},
1
\
2
/
3
return [1,2,3].

List<Integer> result = new ArrayList<Integer>();     public List<Integer> preorderTraversal(TreeNode root) {         resPreorder(root);         return result;        }     private void resPreorder(TreeNode root){         if(root == null) return;         result.add(root.val);         resPreorder(root.left);         resPreorder(root.right);     }