求逆序对

class Solution {
public:
    int result = 0;

    int InversePairs(vector<int> data) {

        int len = data.size();
        vector<int> temp(len);
        MergeSort(data, temp, 0, len-1);
        return result;
    }

    void MergeSort(vector<int>& data, vector<int>& temp, int begin, int end)
    {
        if (begin < end)
        {
            int mid = (end - begin) / 2 + begin;
            MergeSort(data, temp, begin, mid);
            MergeSort(data, temp, mid+1, end);
            MergeArray(data, temp, begin, mid, end);
        } 
    }

    void MergeArray(vector<int>& data, vector<int>& temp, int begin, int mid, int end)
    {
        int i = begin;
        int j = mid + 1;
        int k = 0;

        while (i <= mid && j <= end)
        {
            if (data[i] < data[j])
            {
                temp[k++] = data[i++];
            }
            // 若左半部分当前元素大于右半部分当前元素
            // 则左半部分当前元素后面的每个值都大于它
            else
            {
                result += (mid - i + 1);
                result %=  1000000007;
                temp[k++] = data[j++];
            }
        }

        while (i <= mid)
        {
            temp[k++] = data[i++];
        }

        while (j <= end)
        {
            temp[k++] = data[j++];
        }

        for (i = 0; i < k; i++)
        {
            data[begin+i] = temp[i];
        }
    }
};