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Question: TestDome - Common

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15 February 2018

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/*
Question: TestDome - Common
Solution by Arash Partow 2014

Write a method that finds k-th most common element in a list.

For example, kthMostCommon({ 5, 4, 3, 2, 1, 5, 4, 3, 2, 5, 4, 3, 5,
4, 5 }, 2) should return 4. In the array, element 1 occurs once, 2
twice, 3 three times, 4 four times, and 5 five times, making
element 5 the most common element in the list and 4 the second most
common element.
*/

#include <vector>
#include <stdexcept>
#include <iostream>
#include
#include <unordered_map>

class Common
{
public:

static int kthMostCommon(const std::vector<int>& a, int k)
{
if (k > static_cast<int>(a.size()) || k <= 0)
return -1;

std::unordered_map<int,int> m;

for (auto i : a)
{
m[i]++;
}

std::vector<std::pair<int,int>> n(m.size());

std::copy(m.begin(),m.end(),n.begin());

std::nth_element(n.begin(),n.begin() + (k - 1), n.end(),
[](auto x, auto y){ return y.second < x.second; });

return (n.begin() + (k - 1))->first;
}
};


#ifndef RunTests
int main(int argc, const char* argv[])
{
std::vector<int> input;
input.push_back(1);
input.push_back(2);
input.push_back(2);
input.push_back(3);
input.push_back(3);
input.push_back(3);
input.push_back(4);
input.push_back(4);
input.push_back(4);
input.push_back(4);
input.push_back(5);
input.push_back(5);
input.push_back(5);
input.push_back(5);
input.push_back(5);
int x = Common::kthMostCommon(input, 2);
std::cout x;

return 0;
}
#endif

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