LeetCode "Contains Duplicate II"

class Solution 
{
public:
    bool containsNearbyDuplicate(vector<int>& nums, int k) 
    {
        unordered_set<int> hs;
        for (int i = 0; i < nums.size(); i ++)
        { 
            if (hs.size() > k)
                hs.erase(nums[i - k - 1]);
            if (hs.find(nums[i]) != hs.end())
                return true;
                
            hs.insert(nums[i]);
        }
        return false;
    }
};