Sort elements by frequency using STL

Given an array of integers, sort the array according to frequency of elements. If frequencies of two elements are same, print them in increasing order. Examples:

Input : arr[] = {2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12}
Output : 3 3 3 3 2 2 2 12 12 4 5
Explanation :
No. Freq
2 : 3
3 : 4
4 : 1
5 : 1
12 : 2

We have discussed different approaches in below posts : Sort elements by frequency | Set 1 Sort elements by frequency | Set 2 We can solve this problem using map and pairs. Initially we create a map such that map[element] = freq. Once we are done building the map, we create an array of pairs. A pair which stores elements and their corresponding frequency will be used for the purpose of sorting. We write a custom compare function which compares two pairs firstly on the basis of freq and if there is a tie on the basis of values. 

Below is its c++ implementation : 

Elements sorted by frequency are: 3 3 3 3 2 2 2 12 12 4 5 

Time Complexity : O(n Log n) 

Space Complexity: O(n)
The above algorithm requires O(n) space for the hash map and the array of pairs.