【洛谷P1966】火柴排队

两列排序后将编号一一对应

归并排序求逆序对

（每一次交换就去掉一个逆序对）

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #define ll long long
 5 using namespace std;
 6 const int N=100100;
 7 const ll P=99999997;
 8 ll tmp[N],ss[N],ans,n;
 9 struct fx{
10     ll w;
11     int id;
12 }a[N],b[N];
13 bool cmp(fx p,fx q){
14     return p.w<q.w;
15 }
16 ll read(){
17     ll sum=0;
18     char ch=getchar();
19     while (ch<'0'||ch>'9')
20         ch=getchar();
21     while (ch>='0'&&ch<='9'){
22         sum=sum*10+ch-'0';
23         ch=getchar(); 
24     }
25     return sum;
26 }
27 void mergesort(int l,int r){
28     if(l==r)
29         return;
30     int m=(l+r)>>1;
31     mergesort(l,m);
32     mergesort(m+1,r);
33     int i=l;
34     int j=m+1;
35     int k=l;
36     while (i<=m&&j<=r){
37         if (ss[i]<ss[j]){
38             tmp[k]=ss[i];
39             i++;
40             k++;
41         }
42         else{
43             ans=(ans+m-i+1)%P;//区间内逆序对个数  
44             tmp[k]=ss[j];
45             j++;
46             k++;
47         }
48     }
49     while (i<=m){
50         tmp[k]=ss[i];
51         k++;
52         i++;
53     }
54     while (j<=r){
55         tmp[k]=ss[j];
56         k++;
57         j++;
58     }
59     for (int i=l;i<=r;i++){
60         ss[i]=tmp[i];
61     }
62 }
63 int main(){
64     ans=0;
65     n=read();
66     for (int i=1;i<=n;i++){
67         a[i].w=read();
68         a[i].id=i;
69     }
70     for (int i=1;i<=n;i++){
71         b[i].w=read();
72         b[i].id=i;
73     }
74     sort(a+1,a+n+1,cmp);
75     sort(b+1,b+n+1,cmp);
76     for (int i=1;i<=n;i++)
77         ss[a[i].id]=b[i].id;
78     mergesort(1,n);
79     printf("%lld",ans%P);
80     return 0;
81 }