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 + 10;
const int INF = 0x3f3f3f3f;
int p[MAXN], a[MAXN];
int L[MAXN/2+10], R[MAXN/2+10];
ll ans;

void Merge(int *a, int p, int q, int r)
{
    int n1 = q - p + 1, n2 = r - q;
    for (int i=1; i<=n1; ++i)    L[i] = a[p+i-1];
    for (int i=1; i<=n2; ++i)    R[i] = a[q+i];
    L[n1+1] = INF;    R[n2+1] = INF;

    int i = 1, j = 1;
    for (int k=p; k<=r; ++k)
    {
        if (L[i] <= R[j])    a[k] = L[i++];
        else    {a[k] = R[j++];    ans += n1 - i + 1;}
    }
}

void MergeSort(int *a, int p, int r)
{
    if (p < r)
    {
        int q = (p + r) / 2;
        MergeSort (a, p, q);
        MergeSort (a, q + 1, 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=1; i<=n; ++i)
        {
            int x;    scanf ("%d", &x);    p[x] = i;
        }
        for (int i=1; i<=n; ++i)
        {
            int x;    scanf ("%d", &x);    a[i] = p[x];
        }

        ans = 0;
        MergeSort (a, 1, n);
        printf ("%lld
", ans);
    }

    return 0;
}