Insertion Sort Solutions (InterviewStreet CodeSprint Fall 2011)

Hint.md

The answer is the number of inversions in the array, which is the number of pairs i,j such that i < j and a[i] > a[j]. Counting this is a fairly classical problem with many solutions such as using data structures such as Balanced Trees or Binary Indexed Trees. Another particularly elegant solution involves modifying merge sort to count the number of inversions when merging the two sorted halves in the algorithm.

problemsetter.cpp

#include<iostream>
#include<stdio.h>
#include<string.h>
using namespace std ;
#define MAXN 100002
#define MAX 1000002
int n,a[MAXN],c[MAX] ;
int main()
{
 int runs ;
 scanf("%d",&runs) ;
 while(runs--)
 {
  scanf("%d",&n) ;
  for(int i = 0;i < n;i++) scanf("%d",&a[i]) ;
  long long ret = 1LL * n * (n - 1) / 2 ;
  memset(c,0,sizeof c) ;
  for(int i = 0;i < n;i++)
  {
   for(int j = a[i];j > 0;j -= j & -j) ret -= c[j] ;
   for(int j = a[i];j < MAX;j += j & -j) c[j]++ ;  
  }
  cout << ret << endl ;
 }
 return 0 ;
}

Python version on pythontutor.com, the code visualization might help.

http://goo.gl/uKh8Z

@nma Line 8 should be

for x in range(MAX):

in the code on pythontutor you linked to. Thanks.