逆序对问题(Java实现)归并详解

package com.itwyc;

import java.util.*;

public class Main {
    public static long count = 0;   // 逆序对个数
    public static void getCount(int[] nums) {
        if (nums.length > 1) {
            int[] left_temp = getLeftTemp(nums, 0); // 得到左边数组           
            int[] right_temp = getLeftTemp(nums, 1);// 得到右边数组              
            getCount(left_temp); // 将得到的数组继续拆分                                   
            getCount(right_temp);
            merge(nums, left_temp, right_temp);// 归并排序，归并的同时统计逆序对                     
        }
    }

    public static int[] getLeftTemp(int[] temp, int a) {
        int[] current;
        if (a == 0) {
            current = new int[temp.length / 2];
            for (int i = 0; i < temp.length / 2; i++) {
                current[i] = temp[i];
            }
        } else {
            current = new int[temp.length - temp.length / 2];
            for (int i = 0; i < temp.length - temp.length / 2; i++) {
                current[i] = temp[temp.length / 2 + i];
            }
        }
        return current;
    }

    public static void merge(int[] nums, int[] left_temp, int[] right_temp) {
        int index = 0;
        int left = 0;
        int right = 0;
        int left_len = left_temp.length;
        int right_len = right_temp.length;
        while (left < left_len && right < right_len) {// 如果左边的数大于右边的数，则为逆序对                   
            if (left_temp[left] > right_temp[right]) {
                count += (left_len - left);// 如果此数为逆序对，则此数右边的数也为逆序对，因为两个数组内已经排好序                              
                nums[index++] = right_temp[right++];
            } else {
                nums[index++] = left_temp[left++];
            }
        }
        while (left < left_len) {
            nums[index++] = left_temp[left++];
        }
        while (right < right_len) {
            nums[index++] = right_temp[right++];
        }
    }

    public static void main(String[] args) {

        /**
         *  逆序对问题
         */
        int[] nums = {8, 7, 6, 5, 4, 3, 2, 1};
        getCount(nums);
        System.out.println("逆序对有" + count + "个.");
    }
}