inversion count using BIT

bit_inversion_count.cpp

@@ -0,0 +1,63 @@
+#include <bits/stdc++.h>
+using namespace std;
+int getsum(vector<int>& BIT,int x)
+{  int sum=0;
+    while(x>0)
+    {
+        sum+=BIT[x];
+        x-=(x&(-x));
+    }
+    return sum;
+}
+void update(vector<int>& BIT,int val,int x)
+{
+
+    while(x<BIT.size())
+    {
+        BIT[x]+=val;
+        x+=(x&(-x));
+    }
+}
+int countInversions(vector<int> &A) {
+   int maxi=INT_MIN;
+   vector<int> B,c(A.size());
+   for(int i=0;i<A.size();i++)
+      B.push_back(A[i]);
+    sort(B.begin(),B.end());
+
+   for(int i=0;i<A.size();i++)
+     {
+         int idx=lower_bound(B.begin(),B.end(),A[i])-B.begin();
+         c[i]=idx+1;
+     }
+   for(int i=0;i<c.size();i++)
+     if(maxi<c[i])
+       maxi=c[i];
+    vector<int>BIT(maxi+1,0);
+    int count=0;
+    for(int i=0;i<A.size();i++)
+    {
+        update(BIT,1,c[i]);
+        int a=getsum(BIT,BIT.size()-1);
+        int b=getsum(BIT,c[i]);
+        //cout<<A[i]<<" "<<a<<" "<<b<<"\n";
+        count+=(a-b);
+    }
+    return count;
+}
+
+int main() {
+    //int arr[] = {6,2,1,0,5,4};
+    //int n = sizeof(arr)/sizeof(int);
+    vector<int> A;
+    int n;
+    cin>>n;
+    for(int i=0;i<n;i++)
+    {
+        int a;
+        cin>>a;
+        A.push_back(a);
+    }
+    cout<<countInversions(A)<<endl;
+
+}