puzzle15 c#

using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;

namespace PuzzleNS {
    class Puzzle {
        static int solve(int target, List<int> nums)
        {
            Dictionary<int, int> tab = new Dictionary<int, int>();
            for (int i = 0; i < nums.Count()-1; i++)
                tab.Add(nums[i], i+1);
            int last_num = nums[nums.Count()-1];
            for (int t = nums.Count()+1; t <= target; t++) {
                if (!tab.ContainsKey(last_num)) { /* not found */
                    tab[last_num] = t-1;
                    last_num = 0;
                } else {
                    int newvalue = t-1 - tab[last_num];
                    tab[last_num] = t-1;
                    last_num = newvalue;
                }
            }
            return last_num;
        }

        static void Main(string[] args)
        {
            List<int> nums = File.ReadAllLines("input15.txt")[0].Split(',').ToList().Select(x => int.Parse(x)).ToList();
            Console.WriteLine("part 1: {0}", solve(2020, nums));
            Console.WriteLine("part 2: {0}", solve(30000000, nums));
        }
    }
}