public class Solution
    {
        public bool ContainsNearbyDuplicate(int[] nums, int k)
        {
            //                         value, index
            var dict = new Dictionary<int, int>();
            for (var index = 0; index < nums.Length; index++)
            {
                var i = nums[index];
                if (dict.ContainsKey(i))
                {
                    if (index - dict[i] <= k)
                        return true;
                    dict[i] = index;
                }
                else
                {
                    dict.Add(i, index);
                }
            }

            return false;
        }
    }