public class Solution {
    public bool ContainsNearbyDuplicate(int[] nums, int k) 
    {
        var hash = new Dictionary();
    	for(var i = 0;i < nums.Length; i++){
    		if(!hash.ContainsKey(nums[i])){
    			hash.Add(nums[i],i);
    		}
    		else{
    			if(Math.Abs(hash[nums[i]] - i) <= k){
    				return true;
    			}
    			else{
    				hash[nums[i]] = i;
    			}
    		}
    	}
    	
    	return false;
    }
}