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Codeforces Global Round 13G
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#include<bits/stdc++.h> | |
using namespace std; | |
typedef long long ll; | |
typedef pair<ll,ll> pi; | |
typedef vector<pi> vpi; | |
typedef vector<int> vi; | |
#define ALL(x) x.begin(),x.end() | |
#define f first | |
#define s second | |
#define pb emplace_back | |
#define mp make_pair | |
#define pb emplace_back | |
const int MAXN=200100; | |
vi groups[MAXN]; | |
int N; | |
int A[MAXN]; | |
int vis[MAXN]; | |
int a=1; | |
vpi ans; | |
void dfs(int x,int h){ | |
groups[h].pb(x); | |
vis[x]=1; | |
if(!vis[A[x]])dfs(A[x],h); | |
} | |
void sw(int a,int b){ | |
swap(A[a],A[b]); | |
A[a]=-A[a]; | |
A[b]=-A[b]; | |
ans.pb(a,b); | |
} | |
void res(int x,int l){ | |
if(!l)return; | |
sw(abs(A[x]),x); | |
res(x,l-1); | |
} | |
void fix(int a,int b){ | |
sw(groups[a][0], groups[b][0]); | |
res(groups[b][0],groups[a].size()-1); | |
res(groups[a][0],groups[b].size()-1); | |
sw(groups[a][0], groups[b][0]); | |
} | |
void extfix(int a){ | |
if(groups[a].size()==2){ | |
int oth=-1; | |
for(int i=1;i<=N;++i)if(A[i]==i){ | |
oth=i; | |
} | |
int x1=groups[a][0]; | |
int x2=groups[a][1]; | |
sw(x1,oth); | |
sw(x2,oth); | |
sw(x1,oth); | |
// for(int i=1;i<=N;++i)cout<<A[i]<<' '; | |
return; | |
} | |
int x=groups[a][0]; | |
int elec=A[x]; | |
sw(x,A[x]); | |
// for(int i=1;i<=N;++i)cout<<A[i]<<' ';cerr<<'\n'; | |
res(x,groups[a].size()-3); | |
// exactly 3 things leftover | |
int neg=-1; | |
int pos=-1; | |
for(int i=1;i<=N;++i)if(abs(A[i])!=i){ | |
if(A[i]>0)pos=i; | |
else neg=i; | |
} | |
assert(pos!=-1&&neg!=-1); | |
sw(pos,elec); | |
sw(neg,pos); | |
sw(elec,neg); | |
} | |
int main(){ | |
cin>>N; | |
for(int i=1;i<=N;++i){ | |
cin>>A[i]; | |
} | |
for(int i=1;i<=N;++i){ | |
if(vis[i])continue; | |
if(A[i]==i)continue; | |
dfs(i,a); | |
++a; | |
} | |
--a; | |
for(int i=1;i+1<=a;i+=2){ | |
fix(i,i+1); | |
} | |
if(a%2==1){ | |
extfix(a); | |
} | |
for(int i=1;i<=N;++i)assert(A[i]==i); | |
cout<<ans.size()<<'\n'; | |
for(auto i:ans)cout<<i.f<<' '<<i.s<<'\n'; | |
} |
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