More than 2019 cattle off summer school camp (first)

A.Equivalent+Prefixes

Ideas:

Half the answer, and then use the number to the right of the first stack get monotonous smaller than it is, if for each number to the right of each half of the answer to the left of the first number is smaller than its location must be the same.

 1 #include<bits/stdc++.h>
 2  
 3 using namespace std;
 4  
 5 const int N=200000;
 6 int s[N],tot,a[N],b[N],f1[N],c[N],d[N],f2[N],n;
 7 bool check(int x)
 8 {
 9     for (int i=1; i<=x; i++)
10     {
11         if (f1[i]!=f2[i])
12         {
13             return 0;
14         }
15     }
16     return 1;
17 }
18  
19 void work(int a[],int f[],int d[],int n)
20 {
21     tot=0;
22     for (int i=n; i>=1; i--)
23     {
24         while (tot>0&&s[tot]>a[i])
25         {
26             tot--;
27         }
28         s[++tot]=a[i];
29         if (tot>1)
30         {
31             f[i]=d[s[tot-1]];
32         }
33         else
34         {
35             f[i]=-1;
36         }
37     }
38 }
39  
40 int main()
41 {
42     while(~scanf("%d",&n))
43     {
44         for (int i=1; i<=n; i++)
45         {
46             scanf("%d",&a[i]);
47             c[a[i]]=i;
48         }
49         for (int i=1; i<=n; i++)
50         {
51             scanf("%d",&b[i]);
52             d[b[i]]=i;
53         }
54         int l=1,r=n,ans=1;
55         while (l<=r)
56         {
57             int mid=(l+r)>>1;
58             work(a,f1,c,mid);
59             work(b,f2,d,mid);
60             if (check(mid))
61             {
62                 l=mid+1;
63                 ans=mid;
64             }
65             else
66             {
67                 r=mid-1;
68             }
69         }
70         printf("%d\n",ans);
71     }
72 }

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J