public class Solution {
    public bool ContainsNearbyDuplicate(int[] nums, int k) {
		Dictionary<int, int> dict = new Dictionary<int, int>();
		for (int i = 0; i < nums.Length; i++) {
			int index;
			if (dict.TryGetValue(nums[i], out index)) {
				if (i - index <= k) {
					return true;
				}
			}
			dict[nums[i]] = i;
		}
		return false;
    }
}