一、题目描述

二、算法原理

三、代码实现

3.1升序：

class Solution 
{
public:

    int mergeSort(vector<int>& nums, int left, int right)
    {
        if (left >= right)
        {
            return 0;
        }
        int mid = left + (right - left) / 2, ret = 0;
        ret += mergeSort(nums, left, mid);
        ret += mergeSort(nums, mid + 1, right);

        //求一左一右
        vector<int> temp(right - left + 1);
        int cur1 = left, cur2 = mid + 1, i = 0;
        while (cur1 <= mid && cur2 <= right)
        {
            if (nums[cur1] <= nums[cur2])
            {
                temp[i++] = nums[cur1++];
            }
            else
            {
                ret += (mid - cur1 + 1);
                temp[i++] = nums[cur2++];       
            }
        }

        while (cur1 <= mid) temp[i++] = nums[cur1++];
        while (cur2 <= right) temp[i++] = nums[cur2++];
        for (int i = left; i <= right; i++)
        {
            nums[i] = temp[i - left];
        }
        return ret;
    }



    int reversePairs(vector<int>& record) 
    {
        return mergeSort(record,0,record.size()-1);
    }
};

3.2降序：

class Solution 
{
public:

    int mergeSort(vector<int>& nums, int left, int right)
    {
        if (left >= right)
        {
            return 0;
        }
        int mid = left + (right - left) / 2, ret = 0;
        ret += mergeSort(nums, left, mid);
        ret += mergeSort(nums, mid + 1, right);

        //求一左一右
        vector<int> temp(right - left + 1);
        int cur1 = left, cur2 = mid + 1, i = 0;
        while (cur1 <= mid && cur2 <= right)
        {
            if (nums[cur1] <= nums[cur2])
            {
                temp[i++] = nums[cur2++];
            }
            else
            {
                ret += (right-cur2+1);
                temp[i++] = nums[cur1++];       
            }
        }

        while (cur1 <= mid) temp[i++] = nums[cur1++];
        while (cur2 <= right) temp[i++] = nums[cur2++];
        for (int i = left; i <= right; i++)
        {
            nums[i] = temp[i - left];
        }
        return ret;
    }



    int reversePairs(vector<int>& record) 
    {
        return mergeSort(record,0,record.size()-1);
    }
};