解题思路：

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