模板：最长公共子序列（LCS）

https://www.luogu.org/problemnew/show/P1439

 1 #include<iostream>
 2 #include<cstdio>
 3 using namespace std;
 4 int a[100001], b[100001], map[100001], pos[100001];
 5 int main(){
 6     int n;
 7     cin>>n;
 8     for(int i=1; i<=n; i++){
 9         scanf("%d",&a[i]);
10         map[a[i]]=i;
11     }
12     for(int i=1; i<=n; i++){
13         scanf("%d",&b[i]);
14         pos[i]=0x7fffffff;
15     }
16     int len=0;
17     pos[0]=0;
18     for(int i=1; i<=n; i++){
19         int l=0, r=len, mid;
20         if(map[b[i]]>pos[len])  pos[++len]=map[b[i]];
21         else{
22             while(l<r){   
23                     mid=(l+r)/2;
24                    if(pos[mid]>map[b[i]] ) r=mid;
25                     else l=mid+1; 
26             }
27             pos[l]=min(map[b[i]], pos[l]);
28         }
29     }
30     cout<<len<<endl;
31     return 0;
32 }