BM20.java

package NiukeTOP101;

public class BM20 {
    public int mod = 1000000007;
    public int mergeSort(int left, int right, int [] data, int [] temp){
        //停止划分
        if(left >= right)
            return 0;
        //取中间
        int mid = (left + right) / 2;
        //左右划分合并
        int res = mergeSort(left, mid, data, temp) + mergeSort(mid + 1, right, data, temp);
        //防止溢出
        res %= mod;
        int i = left, j = mid + 1;
        for(int k = left; k <= right; k++)
            temp[k] = data[k];
        for(int k = left; k <= right; k++){
            if(i == mid + 1)
                data[k] = temp[j++];
            else if(j == right + 1 || temp[i] <= temp[j])
                data[k] = temp[i++];
                //左边比右边大，答案增加
            else{
                data[k] = temp[j++];
                // 统计逆序对
                res += mid - i + 1;
            }
        }
        return res % mod;
    }

    public int InversePairs(int [] array) {
        int n = array.length;
        int[] res = new int[n];
        return mergeSort(0, n - 1, array, res);
    }

}