求逆序对个数 

 1 ll a[amn],tmpA[amn],cnt=0;
 2 void merge_sort(ll l, ll r, ll *A) {
 3     if (l >= r) return ;
 4     int mid = (l + r) >> 1;
 5     merge_sort(l, mid, A);
 6     merge_sort(mid + 1, r, A);
 7     int pl = l, pr = mid + 1, tmpp = 0;
 8     while(pl <= mid && pr <= r) {
 9         if (A[pl] <= A[pr]) tmpA[tmpp++] = A[pl++];
10         else tmpA[tmpp++] = A[pr++], cnt += mid - pl + 1;
11     }
12     while(pl <= mid) tmpA[tmpp++] = A[pl++];
13     while(pr <= r) tmpA[tmpp++] = A[pr++];
14     for (int i = 0; i < tmpp; i++) A[i + l] = tmpA[i];
15 }

cnt 保存逆序对个数

posted @ 2020-09-30 20:11  Railgun000  阅读(169)  评论(0)  编辑  收藏  举报