LeetCode 219. Contains Duplicate II

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