44 lines
1.0 KiB
Java
44 lines
1.0 KiB
Java
|
import java.util.*;
|
||
|
|
|
||
|
|
public class Inversions {
|
||
|
|
|
||
|
|
// warning: a is modified during processing
|
||
|
|
public static long inversions(int[] a, int low, int high) {
|
||
|
|
if (high - low < 2)
|
||
|
|
return 0;
|
||
|
|
int mid = (low + high) >>> 1;
|
||
|
|
long res = inversions(a, low, mid) + inversions(a, mid, high);
|
||
|
|
int[] b = Arrays.copyOfRange(a, low, mid);
|
||
|
|
for (int i = low, j = mid, k = 0; k < b.length; i++)
|
||
|
|
if (j == high || b[k] <= a[j]) {
|
||
|
|
a[i] = b[k++];
|
||
|
|
} else {
|
||
|
|
a[i] = a[j++];
|
||
|
|
res += b.length - k;
|
||
|
|
}
|
||
|
|
return res;
|
||
|
|
}
|
||
|
|
|
||
|
|
// random test
|
||
|
|
public static void main(String[] args) {
|
||
|
|
Random rnd = new Random(1);
|
||
|
|
for (int step = 0; step < 1000; step++) {
|
||
|
|
int n = rnd.nextInt(100) + 1;
|
||
|
|
int[] p = rnd.ints(n, 0, n).toArray();
|
||
|
|
long res1 = inversions(p.clone(), 0, p.length);
|
||
|
|
long res2 = slowInversions(p);
|
||
|
|
if (res1 != res2)
|
||
|
|
throw new RuntimeException();
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
static long slowInversions(int[] p) {
|
||
|
|
long res = 0;
|
||
|
|
for (int i = 0; i < p.length; i++)
|
||
|
|
for (int j = 0; j < i; j++)
|
||
|
|
if (p[j] > p[i])
|
||
|
|
++res;
|
||
|
|
return res;
|
||
|
|
}
|
||
|
|
}
|