class Solution {
public:
    typedef long long ll;
    vector<int> tmp;
    ll merge_sort(vector<int>& nums, int l, int r){
        if(l == r)  return 0;
        int mid = l + r >> 1;
        ll res = merge_sort(nums, l, mid) + merge_sort(nums, mid + 1, r);
        int i = l, j = mid + 1;
        while(i <= mid && j <= r){
            if(nums[i] <= nums[j])    tmp.push_back(nums[i++]);
            else{
                res += mid - i + 1;
                tmp.push_back(nums[j++]);
            }
        }
        while(i <= mid)     tmp.push_back(nums[i++]);
        while(j <= r)   tmp.push_back(nums[j++]);
        int k = 0;
        for(i = l; i <= r; i ++ ){
            nums[i] = tmp[k++];
        }
        tmp.clear();
        return res;
    }
    int inversePairs(vector<int>& nums) {
        if(nums.size() == 0)    return 0;
        return merge_sort(nums, 0, nums.size()-1);
    }
};