exclude non-duplicated int in vector - c++

Question

I'm new to C++, I came from Swift background, and I'm thankful for your help in advance.
I have a vector that contains int values. Some of them are repeated.
My task here is to get the largest repeated value from the vector.
Example:
std::vector<int> myVector;
myVector.push_back(1);
myVector.push_back(8);
myVector.push_back(4);
myVector.push_back(4);
I need a function that returns 4 because it's the largest duplicate int in the vector.
Thanks again, and please, if you have any question, please ask it instead of downvoting.

Answer 1

Solution based only on std algorithms:
Sort the list using std::sort.
Iterate backwards over its elements and detect the first one that is equal to its predecessor using std::adjacent_find and reverse iterators.
I doubt it gets simpler than this. For your enjoyment:
#include <algorithm>
#include <iostream>
#include <vector>
int main()
{
std::vector<int> v{1,2,3,3,4,4,4,5,6,7,7,8};
std::sort(v.begin(), v.end());
auto result = std::adjacent_find(v.rbegin(), v.rend());
if(result == v.rend())
std::cout << "No duplicate elements found.";
else
std::cout << "Largest non-unique element: " << *result;
}
Live example on Coliru.
Properties:
Zero space overhead if the list can be sorted in place.
Complexity: O(N log(N)) less than comparisons and K equality comparisons where K is equal to the number of unique elements larger than the one you're after.
Lines of code making up the algorithm: 2

Answer 2

You could use a map, as someone who commented above, and then place the number of appearances of each element of the vector. Afterwards, you take the maximum element via a custom comparator.
Ideone
#include <iostream>
#include <algorithm>
#include <map>
#include <vector>
int largestRepeatingNumber(const std::vector<int> & vec)
{
std::map<int, int> counter;
std::for_each(std::begin(vec), std::end(vec), [&counter] (int elem) {
counter.find(elem) == counter.end() ? counter[elem] = 1 : ++counter[elem]; });
return std::max_element(std::begin(counter), std::end(counter), [] (auto lhs, auto rhs) {
if (lhs.second == rhs.second)
return lhs.first < rhs.first;
return lhs.second < rhs.second;
})->first;
}
int main()
{
std::vector<int> myVector;
myVector.push_back(1);
myVector.push_back(8);
myVector.push_back(4);
myVector.push_back(4);
myVector.push_back(3);
myVector.push_back(3);
std::cout << largestRepeatingNumber(myVector);
return 0;
}

Answer 3

I have used the lower bound and upper bound
so strategy is
1)Sort the original vector (so that can use unique function on it)
2)find unique (copy the unique values to a vector so that we can use it to find the values in original vector , not extra search)
3)Find the lower and upper bound value with max distance
for example 1 4 4 8
4 will have max distance
5)Store in map using the count as index (map is ordered so max duplicate value will be at the end )
#include<iostream>
#include<algorithm>
#include<vector>
#include<map>
using namespace std;
int main()
{
std::vector<int> myVector,myvec,counter;
map<int,int> maxdupe;
myVector.push_back(1);
myVector.push_back(8);
myVector.push_back(4);
myVector.push_back(4);
sort(myVector.begin(),myVector.end());
std::unique_copy(myVector.begin(), myVector.end(), std::back_inserter(myvec));
std::copy(myvec.begin(), myvec.end(), std::ostream_iterator<int>(std::cout, " "));
cout<<endl;
for(auto i = myvec.begin(); i!=myvec.end();i++)
{
auto lower = std::lower_bound(myVector.begin(), myVector.end(), *i);
auto upper = std::upper_bound(myVector.begin(), myVector.end(), *i);
maxdupe[upper - lower] = *i;
}
for(auto i= maxdupe.begin();i!= maxdupe.end();i++)
{
cout<<i->first<<i->second<<endl;
}
return 0;
}
Output
1 4 8
18
24
Program ended with exit code: 0