22E - Scheme - CodeForces Solution

N=int(input())
A=list(map(int,input().split()))

E=[[] for i in range(N)]
E_INV=[[] for i in range(N)]

for i in range(N):
    x,y=i,A[i]-1
    E[x].append(y)
    E_INV[y].append(x)

def Top_sort(E):
    Parent=[-1]*N
    USEIND=[0]*N
    TOP=[]

    for ROOT in range(N):
        if Parent[ROOT]!=-1:
            continue
        Parent[ROOT]=ROOT

        NOW=ROOT

        while NOW!=ROOT or USEIND[ROOT]!=len(E[ROOT]):

            if USEIND[NOW]==len(E[NOW]):
                TOP.append(NOW)
                NOW=Parent[NOW]
            elif E[NOW][USEIND[NOW]]==Parent[NOW]:
                USEIND[NOW]+=1
            else:
                NEXT=E[NOW][USEIND[NOW]]
                USEIND[NOW]+=1
                if Parent[NEXT]==-1:
                    Parent[NEXT]=NOW
                    NOW=NEXT
        TOP.append(ROOT)
        
    return TOP[::-1]


USE=[0]*N
SCC=[]

def dfs2(x):
    Q=[x]
    USE[x]=1
    ANS=[]

    while Q:
        x=Q.pop()
        ANS.append(x)
        for to in E_INV[x]:
            if USE[to]==0:
                USE[to]=1
                Q.append(to)
    return ANS

TOP_SORT=Top_sort(E)

for x in TOP_SORT:
    if USE[x]==0:
        SCC.append(dfs2(x))


Group = [i for i in range(N+1)] Nodes = [1]*(N+1) 
def find(x):
    while Group[x] != x:
        x=Group[x]
    return x

def Union(x,y):
    if find(x) != find(y):
        if Nodes[find(x)] < Nodes[find(y)]:
            
            Nodes[find(y)] += Nodes[find(x)]
            Nodes[find(x)] = 0
            Group[find(x)] = find(y)
            
        else:
            Nodes[find(x)] += Nodes[find(y)]
            Nodes[find(y)] = 0
            Group[find(y)] = find(x)

for i in range(N):
    Union(i+1,A[i])

LAST=[]
USE=[0]*(N+1)
for scc in SCC[::-1]:
    for x in scc[::-1]:
        if USE[find(x+1)]==0:
            USE[find(x+1)]=1
            LAST.append(x+1)

FIRST=[]
USE=[0]*(N+1)
for scc in SCC:
    for x in scc:
        if USE[find(x+1)]==0:
            USE[find(x+1)]=1
            FIRST.append(x+1)

USE=[0]*(N+1)
for a in A:
    USE[a]=1

ANS=[]
ind=1

FIRST.sort(key=lambda x:find(x))
LAST.sort(key=lambda x:find(x))

if len(LAST)!=1:
    for i in FIRST:
        if ind==len(LAST):
            ind=0
        ANS.append((LAST[ind],i))
        Union(LAST[ind],i)
        USE[i]+=1
        USE[LAST[ind]]+=1

        ind+=1
if ind==len(LAST):
    ind=0

    
for i in range(1,N+1):
    if USE[i]==0:
        while Nodes[find(i)]<N and find(i)==find(LAST[ind]):
            ind+=1
            if ind==len(LAST):
                ind=0
                
        ANS.append((LAST[ind],i))
        Union(LAST[ind],i)




print(len(ANS))
for x,y in ANS:
    print(x,y)