Clean, Short Iterative Solution to PreOrder

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    public class Solution {
        public List<Integer> preorderTraversal(TreeNode root) {
            Stack<TreeNode> st = new Stack<>();
            List<Integer> ans = new ArrayList<>();
            if (root == null) return ans;
                TreeNode temp = st.pop();
                if(temp.right != null) st.push(temp.right);
                if(temp.left != null) st.push(temp.left);
            return ans;

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