Given a binary tree, return thepreordertraversal 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?

tag: binary tree

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

        dfs(root, ans);

        return ans;
    }

    private void dfs(TreeNode root, List<Integer> ans){
        if (root == null) return;

        ans.add(root.val);
        dfs(root.left, ans);
        dfs(root.right, ans);
    }
}

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

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);

        while (!stack.isEmpty()){
            TreeNode cur = stack.pop();
            ans.add(cur.val);

            if (cur.right != null) stack.push(cur.right);
            if (cur.left != null) stack.push(cur.left);
        }

        return ans;
    }
}