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countInversion.cpp
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countInversion.cpp
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#include <bits/stdc++.h>
void merge(long long *arr, long long begin, long long mid, long long end, long long& result) {
long long leftArraySize = mid - begin + 1;
long long rightArraySize = end - mid;
long long *leftArray = new long long[leftArraySize];
long long *rightArray = new long long[rightArraySize];
long long k = 0;
for(int i=0;i<leftArraySize;i++) {
leftArray[i] = arr[begin+i];
}
for(int i=0;i<rightArraySize;i++) {
rightArray[i] = arr[mid+1+i];
}
int index1 = 0, index2 = 0, arrPointer = begin;
while(index1<leftArraySize && index2<rightArraySize) {
if(leftArray[index1] <= rightArray[index2]) {
arr[arrPointer++] = leftArray[index1++];
}
else {
arr[arrPointer++] = rightArray[index2++];
result+=(leftArraySize-index1);
}
}
while(index1<leftArraySize) {
arr[arrPointer++] = leftArray[index1++];
}
while(index2<rightArraySize) {
arr[arrPointer++] = rightArray[index2++];
}
}
void mergeSort(long long *arr, long long begin, long long end, long long& result) {
if(begin < end) {
int mid = begin + (end-begin)/2;
mergeSort(arr, begin, mid, result);
mergeSort(arr, mid+1, end, result);
merge(arr, begin, mid, end, result);
}
}
long long getInversions(long long *arr, int n){
// Write your code here.
long long result = 0;
mergeSort(arr, 0, n-1, result);
// for(int i=0;i<n;i++) {
// cout<<arr[i]<<" ";
// }
return result;
}