逆序数 UVALive 6508 Permutation Graphs

题目传送门

 /*
     题意：给了两行的数字，相同的数字连线，问中间交点的个数
     逆序数：第一行保存每个数字的位置，第二行保存该数字在第一行的位置，接下来就是对它求逆序数
         用归并排序或线段树求。想到了就简单了:)
 */
 #include <cstdio>
 #include <algorithm>
 #include <cstring>
 #include <cmath>
 #include <vector>
 using namespace std;

 typedef long long ll;
 const int MAXN = 1e5 + ;
 const int INF = 0x3f3f3f3f;
 int p[MAXN], a[MAXN];
 int L[MAXN/+], R[MAXN/+];
 ll ans;

 void Merge(int *a, int p, int q, int r)
 {
     int n1 = q - p + , n2 = r - q;
     for (int i=; i<=n1; ++i)    L[i] = a[p+i-];
     for (int i=; i<=n2; ++i)    R[i] = a[q+i];
     L[n1+] = INF;    R[n2+] = INF;

     int i = , j = ;
     for (int k=p; k<=r; ++k)
     {
         if (L[i] <= R[j])    a[k] = L[i++];
         else    {a[k] = R[j++];    ans += n1 - i + ;}
     }
 }

 void MergeSort(int *a, int p, int r)
 {
     if (p < r)
     {
         int q = (p + r) / ;
         MergeSort (a, p, q);
         MergeSort (a, q + , r);
         Merge (a, p, q, r);
     }
 }

 int main(void)        //UVALive 6508 Permutation Graphs
 {
 //    freopen ("I.in", "r", stdin);

     int t;    scanf ("%d", &t);
     while (t--)
     {
         int n;    scanf ("%d", &n);
         for (int i=; i<=n; ++i)
         {
             int x;    scanf ("%d", &x);    p[x] = i;
         }
         for (int i=; i<=n; ++i)
         {
             int x;    scanf ("%d", &x);    a[i] = p[x];
         }

         ans = ;
         MergeSort (a, , n);
         printf ("%lld\n", ans);
     }

     return ;
 }