https://codeforces.com/problemset/problem/1148/C
Idea: It is found that 1 and n are special, and the numbers are transferred here every time through 1 and n as the transfer station.
<=n/2, the number on the left must be exchanged with n, and then exchanged to 1. <=n/2 on the right, exchange with 1, then exchange with n, and then exchange with i.
>=n/2+1 the same
#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=3e5+1000;
typedef long long LL;
typedef pair<LL,LL>P;
inline LL read(){LL x=0,f=1;char ch=getchar(); while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
LL a[maxn],pos[maxn];
vector<P>ans;
void op(LL x,LL y){
ans.push_back({x,y});
swap(pos[a[x]],pos[a[y]]);
swap(a[x],a[y]);
}
int main(void)
{
cin.tie(0);std::ios::sync_with_stdio(false);
LL n;cin>>n;
for(LL i=1;i<=n;i++){
cin>>a[i];
pos[a[i]]=i;
}
for(LL i=2;i<=n/2;i++){
if(pos[i]<=n/2){
op(pos[i],n);
op(n,i);
}
else{
op(pos[i],1);
op(1,n);
op(n,i);
}
}
for(LL i=n/2+1;i<n;i++){
if(pos[i]<=n/2){
op(pos[i],n);
op(n,1);
op(1,i);
}
else{
op(pos[i],1);
op(1,i);
}
}
if(pos[1]!=1) op(1,n);
cout<<ans.size()<<"\n";
for(auto i:ans){
cout<<i.first<<" "<<i.second<<"\n";
}
return 0;
}