Sort elements by frequency.cpp

 #include <bits/stdc++.h>
using namespace std;

// Used for sorting by frequency. And if frequency is same,
// then by appearance
bool sortByVal(const pair<int, int>& a, 
					const pair<int, int>& b)
{

// If frequency is same then sort by index
if (a.second == b.second) 
	return a.first < b.first;

return a.second > b.second;
}

// function to sort elements by frequency
vector<int>sortByFreq(int a[], int n)
{

vector<int>res;

unordered_map<int, int> m;

vector<pair<int, int> > v;

for (int i = 0; i < n; ++i) {

	// Map m is used to keep track of count 
	// of elements in array
	m[a[i]]++;	 
}

// Copy map to vector
copy(m.begin(), m.end(), back_inserter(v));

// Sort the element of array by frequency
sort(v.begin(), v.end(), sortByVal);

for (int i = 0; i < v.size(); ++i) 
	while(v[i].second--)
	{
			res.push_back(v[i].first);
	}

return res;
}

// Driver program
int main()
{

int a[] = { 2, 5, 2, 6, -1, 9999999, 5, 8, 8, 8 };
int n = sizeof(a) / sizeof(a[0]);
vector<int>res;
res = sortByFreq(a, n);

for(int i = 0;i < res.size(); i++)
		cout<<res[i]<<" ";

return 0;

}