Preorder iterative

PreorderIterative.java

package com.interviewbytes.trees;

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.List;

public class PreorderIterative {

    public List<Integer> preorderTraversal(TreeNode root) {

        List<Integer> list = new ArrayList<>();

        Deque<TreeNode> stack = new ArrayDeque<>();
        if (root != null) stack.push(root);

        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.right != null) stack.push(node.right);
            if (node.left != null) stack.push(node.left);

            list.add(node.val);
        }

        return list;
    }
}

TreeNode.java

package com.interviewbytes.trees;

public class TreeNode {

    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}