hdu 4911 Inversion 多校第五场 求逆序数

//187MS	1080K
#include <cstdio>
#include <iostream>
#include <cstring>
using namespace std;

const int N = int(1e5) + 5;
long long cnt, k;
int a
,c
;

void merge(int a[], int first, int mid, int last, int c[]){
int i = first, j = mid + 1;
int m = mid, n = last;
int k = 0;
while(i <= m || j <= n){
if(j > n || (i <= m && a[i] <= a[j]))
c[k++] = a[i++];
else{
c[k++] = a[j++];
cnt += (m - i + 1);
}
}
for(i = 0; i < k; i++)
a[first + i] = c[i];
}

void merge_sort(int a[], int first, int last, int c[]){
if(first < last){
int mid = (first + last) / 2;
merge_sort(a, first, mid, c);
merge_sort(a, mid+1, last, c);
merge(a, first, mid, last, c);
}
}

int main(){
int n;
while(~scanf("%d%I64d", &n, &k)){
memset(c, 0, sizeof(c));
cnt = 0;
for(int i = 0; i < n; i++)
scanf("%d", &a[i]);
merge_sort(a, 0, n-1, c);
if(k >= cnt) cnt = 0;
else cnt -= k;
printf("%I64d\n",cnt);
}
return 0;
}