219.cpp

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