归并排序(逆序对数)

ll qans;           // 记录答案  int date[maxn];    // 数据数组  int tarray[maxn];  // 归并排序的中间数组   // POJ - 2299     SGU - 180 void merge(int arr[], int left, int right, int tarr[])  {     if (left>=right) return ;     int m = (left+right) >> 1;     merge(arr, left, m, tarr);     merge(arr, m+1, right, tarr);     int i,j, cnt;     i = left;     j = m+1;     cnt = 0;     while (i<=m && j<=right) {         if (arr[i] <= arr[j])              tarr[cnt++] = arr[i++];         else {             qans += (ll)m - i + 1;  // 求逆序对.              tarr[cnt++] = arr[j++];          }     }     while (i<=m) tarr[cnt++] = arr[i++];     while (j<=right) tarr[cnt++] = arr[j++];     for (i=0; i<cnt; ++i) arr[left++] = tarr[i];  }

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原文：https://www.cnblogs.com/cgjh/p/9347674.html