Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j] and the absolute difference between i and j is at most k.

1. use a set to keep track of the n seen in the k window, update the set by adding the new n and removing the oldest n.

**********62ms
class Solution(object):
    def containsNearbyDuplicate(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: bool
        """
        if k <= 0:
            return False
        l = len(nums)
        s = set()
        i = 0
        while i<=k and i<l:
            if nums[i] in s:
                return True
            else:
                s.add(nums[i])
            i += 1
        while i<l:
            s.remove(nums[i-k-1])
            if nums[i] in s:
                return True
            else:
                s.add(nums[i])
            i += 1
        return False

from other ppl's solution:
python's set and dict conversion are unblievable cheap.
1. brutal force is faster, 35ms:
class Solution(object):
    def containsNearbyDuplicate(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: bool
        """
        dict1 = {}
        for i, v in enumerate(nums):
            if v not in dict1:
                dict1[v] = i
            else:
                if i - dict1[v] <= k:
                    return True
                else:
                    dict1[v] = i
        return False