144. Binary Tree Preorder Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new LinkedList<>();
        
        Deque<TreeNode> stack = new LinkedList<>();
        TreeNode node = root;
        while(node!=null || stack.size()>0){
            if(node!=null){
                result.add(node.val);
                if(node.right!=null) stack.push(node.right);
                if(node.left!=null) stack.push(node.left);
            }
            
            if(stack.size()>0) node=stack.pop();
            else node = null;
        }
        return result;
    }
}