codeforces1283F

题目链接

sol：根一定是第一个， 然后不太会，去看了洛谷题解

题解

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int,int> pii;
#define fi first
#define se second
#define fz1(i,n) for ((i)=1;(i)<=(n);(i)++)
#define fd1(i,n) for ((i)=(n);(i)>=1;(i)--)
#define fz0g(i,n) for ((i)=0;(i)<=(n);(i)++)
#define fd0g(i,n) for ((i)=(n);(i)>=0;(i)--)
#define fz0k(i,n) for ((i)=0;(i)<(n);(i)++)
#define fd0k(i,n) for ((i)=(((long long)(n))-1);(i)>=0;(i)--)
#define fz(i,x,y) for ((i)=(x);(i)<=(y);(i)++)
#define fd(i,y,x) for ((i)=(y);(i)>=(x);(i)--)
#define ff(c,itr) for (__typeof((c).begin()) itr=(c).begin();itr!=(c).end();++itr)
#define pb push_back
#define mp make_pair
#define inf LLONG_MAX
#define iinf INT_MAX

const char al='\n';

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
#ifndef ONLINE_JUDGE
    freopen("test.in","r",stdin);
#endif

    int n,i;
    cin>>n;
    vector<int>a(n+5);
    vector<bool>jyl(n+5);
    fz1(i,n-1) cin>>a[i];
    cout<<a[1]<<al; jyl[a[1]]=1;
    int index=n;
    fz1(i,n-1) {
        while(jyl[index]) index--;
        if(jyl[a[i+1]]||i==n-1) {
            cout<<a[i]<<" "<<index<<al; jyl[index]=1;
        } else {
            cout<<a[i]<<" "<<a[i+1]<<al; jyl[a[i+1]]=1;
        }
    }

    return 0;
}