计算逆序对

归并排序(计算逆序对)

import java.io.BufferedInputStream;
import java.util.Scanner;

public class test06 {
	static int count = 0;

	public static void main(String[] args) {
		Scanner scan = new Scanner(new BufferedInputStream(System.in));
		int n = scan.nextInt();
		scan.nextLine();
		String[] str = scan.nextLine().split(" ");
		int[] num = new int[str.length];
		for (int i = 0; i < num.length; i++) {
			num[i] = Integer.parseInt(str[i]);
		}
		sort(num, 0, num.length);//归并排序
		System.out.println(count);
	}

	static void sort(int[] num, int begin, int end) {
		int temp = (end - begin) / 2;
		if (temp > 0) {
			sort(num, begin, end - temp);
			sort(num, end - temp, end);
		}
		megre(num, begin, end - temp, end - temp, end);
	}

	static void megre(int[] num, int b1, int e1, int b2, int e2) {
		int[] temp = new int[e2 - b1];
		int i = 0, m = b1, n = b2;
		for (; m < e1 && n < e2; i++) {
			if (num[m] < num[n]) {
				temp[i] = num[m];
				m++;
			} else {
				count = count + e1 - m;//根据题意计算的逆序对
				temp[i] = num[n];
				n++;
			}
		}
		for (int j = i; j < temp.length; j++) {
			if (m < e1) {
				temp[j] = num[m];
				m++;
			}
			if (n < e2) {
				temp[j] = num[n];
				n++;
			}
		}
		for (int j = 0; j < temp.length; j++) {
			num[j + b1] = temp[j];
		}

	}
}