Contains Duplicate II

[code]class Solution {
public:
    bool containsNearbyDuplicate(vector<int>& nums, int k) {
        map<int, int> hashRecord;
        int length = nums.size();
        for(int i = 0; i < length; i++) {
            if(hashRecord.count(nums[i]) && i - hashRecord[nums[i]] <= k)
                return true;
            else
                hashRecord[nums[i]] = i;
        }
        return false;
    }
};