-
Notifications
You must be signed in to change notification settings - Fork 1
/
Inversions.java
61 lines (55 loc) · 1.57 KB
/
Inversions.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
import java.util.*;
public class Inversions {
private static long merge(int[] a, int[] l, int[] r, int left, int right) {
long sum = 0;
int mid = (left + right) / 2;
int i = 0, j = 0, k = 0;
while (i < left && j < right) {
if (l[i] <= r[j]) {
a[k++] = l[i++];
} else {
a[k++] = r[j++];
sum += mid - i;
}
}
while (i < left) {
a[k++] = l[i++];
}
while (j < right) {
a[k++] = r[j++];
}
return sum;
}
public static long mergeSort(int[] a, int n) {
if (n < 2) {
return 0;
}
long sum = 0;
int mid = n / 2;
int[] l = new int[mid];
int[] r = new int[n - mid];
for (int i = 0; i < mid; i++) {
l[i] = a[i];
}
for (int i = mid; i < n; i++) {
r[i - mid] = a[i];
}
sum += mergeSort(l, mid);
sum += mergeSort(r, n - mid);
sum += merge(a, l, r, mid, n - mid);
return sum;
}
private static long getNumberOfInversions(int[] a, int[] b, int left, int right) {
return mergeSort(a, right);
}
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
int[] a = new int[n];
for (int i = 0; i < n; i++) {
a[i] = scanner.nextInt();
}
int[] b = new int[n];
System.out.println(getNumberOfInversions(a, b, 0, a.length));
}
}