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

Example 1:

Input: nums = [1,2,3,1], k = 3
Output: true

Example 2:

Input: nums = [1,0,1,1], k = 1
Output: true

Example 3:

Input: nums = [1,2,3,1,2,3], k = 2
Output: false

思路：用unordered_set，並且始終保持set中的元素在k個以內，一旦超過k，就刪除前面的元素，這樣保證了set中不會有下標距離之差大於k的元素。

程式碼如下：

    bool containsNearbyDuplicate(vector<int>& nums, int k) {
            unordered_set<int> s;
            if(k<=0)
                return false;
            if(k>=nums.size())
                k=nums.size()-1;
            
            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;
        }