题解 | #数组中的逆序对#

class Solution {
public:
    void merge_sort(vector<int>& vec, int l, int r, int& res) {
        if(l >= r)
            return;
        int mid = (l + r) / 2;
        merge_sort(vec, l, mid, res);
        merge_sort(vec, mid + 1, r, res);
        int i = l, j = mid + 1, k = 0;
        vector<int> temp(r-l+1);
        while(i <= mid && j <= r) {
            if(vec[i] <= vec[j]) {
                temp[k++] = vec[i++];
            }else {
                res += (mid - i + 1);
                res = res % 1000000007;
                temp[k++] = vec[j++];
            }
        }
        while(i <= mid) {
            temp[k++] = vec[i++];
        }
        while(j <= r) {
            temp[k++] = vec[j++];
        }
        i = l, k = 0;
        while(l <= r) {
            vec[l++] = temp[k++];
        }
        return;
    }
    int InversePairs(vector<int> data) {
        int res = 0;
        int n = data.size();
        if( n < 2)
            return res;
        merge_sort(data, 0, n - 1, res);
        return res;
    }
};