https://codeforces.com/problemset/problem/1148/C

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思路:发现1和n比较特殊，通过1和n作为中转站每次把数字在这里转。

<=n/2的要在左边的数字先和n换，再换到1. <=n/2的在右边的先和1换，再和n换，再和i换。

>=n/2+1同理

#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;
}