题解 | #数组中的逆序对#

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param nums int整型一维数组 
     * @return int整型
     */
     public int mod = 1000000007;
     public int merge(int first, int end, int[] nums, int[] arr){
        if(first >= end){
            return 0;
        }
        int mid = first + ((end - first)>>1);
        int res = merge(first,mid,nums,arr) + merge(mid+1,end,nums,arr);
        res %= mod;
        int i = first, j = mid + 1;
        for(int k = first; k <= end; k++){
            arr[k] = nums[k];
        }
        for(int k = first; k <= end; k++){
            if(i == mid + 1){
                nums[k] = arr[j++];
            }else if(j == end + 1 || arr[i] <= arr[j]){
                nums[k] = arr[i++];
            }else{
                nums[k] = arr[j++];
                res += mid - i + 1;
            }
        }
        return res%mod;
     }
    public int InversePairs (int[] nums) {
        // write code here
        int n = nums.length;
        int[] arr = new int[n];
        return merge(0,n-1,nums,arr);
    }
}