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Counting Inversions using Set in C++ STL
In this tutorial, we will be discussing a program to count inversions using set in C++ STL.
Inversion count is a measure of how near the array is to be completely sorted. If the array is already sorted, the inversion count will be 0.
Example
#include<bits/stdc++.h>
using namespace std;
//returning inversion count
int get_Icount(int arr[],int n){
multiset<int> set1;
set1.insert(arr[0]);
int invcount = 0; //initializing result
multiset<int>::iterator itset1;
for (int i=1; i<n; i++){
set1.insert(arr[i]);
itset1 = set1.upper_bound(arr[i]);
invcount += distance(itset1, set1.end());
}
return invcount;
}
int main()
{
int arr[] = {8, 4, 2, 1};
int n = sizeof(arr)/sizeof(int);
cout << "Number of inversions count are : "<< get_Icount(arr,n);
return 0;
}Output
Number of inversions count are : 6
Updated on: 16-Mar-2020
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