[자료구조] Binary Tree Preorder Traversal

바이너리 트리에서 전위순회 구현해보기
Given a binary tree, return the preorder traversal of its nodes’ values.

For example:
Given binary tree [1,null,2,3],

   1
    \
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

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