Contains Duplicate II leetcode

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 difference between i and jis at most k.

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bool containsNearbyDuplicate(vector<int>& nums, int k) {
    unordered_map<int, int> hash;
    int i = 0;
    for (auto num : nums)
    {
        unordered_map<int, int>::iterator iter = hash.find(num);
        if (iter == hash.end())
            hash.insert(make_pair(num, i));
        else {
            if (i - (*iter).second <= k)
                return true;
            (*iter).second = i;
        }
        ++i;
    }
    return false;
}