Contains Duplicate II

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

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