0144-Binary-tree-preorder-traversal.cs

using Common;
using System;
using System.Collections.Generic;
using System.Text;

namespace Solution._0144.Binary_tree_preorder_traversal
{
    public class _0144_Binary_tree_preorder_traversal
    {
        /// <summary>
        /// Recursive solution
        /// </summary>
        /// <param name="root"></param>
        /// <returns></returns>
        public IList<int> PreorderTraversal(TreeNode root)
        {
            IList<int> res = new List<int>();

            if (root == null) return res;

            PreOrder(root, ref res);

            return res;
        }

        private void PreOrder(TreeNode node, ref IList<int> res)
        {
            if (node != null)
            {
                res.Add(node.val);
                PreOrder(node.left, ref res);
                PreOrder(node.right, ref res);
            }
        }

        /// <summary>
        /// Iterativeley solution
        /// </summary>
        /// <param name="root"></param>
        /// <returns></returns>
        //public IList<int> PreorderTraversal(TreeNode root)
        //{
        //    IList<int> result = new List<int>();

        //    if (root == null) return result;

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

        //    while (stack.Count > 0)
        //    {
        //        TreeNode current = stack.Pop();

        //        if (current != null)
        //        {
        //            result.Add(current.val);
        //            stack.Push(current.right);
        //            stack.Push(current.left);
        //        }
        //    }

        //    return result;
        //}
    }
}