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;
}