codeforces 1363C（优先队列）

#include<bits/stdc++.h>
#define x first
#define y second
#define PB push_back
#define mst(x,a) memset(x,a,sizeof(x))
#define all(a) begin(a),end(a)
#define rep(x,l,u) for(ll x=l;x<u;x++)
#define rrep(x,l,u) for(ll x=l;x>=u;x--)
#define sz(x) x.size()
#define IOS ios::sync_with_stdio(false);cin.tie(0);
using namespace std;
typedef unsigned long long ull;
typedef pair<int,int> PII;
typedef pair<long,long> PLL;
typedef pair<char,char> PCC;
typedef long long ll;
const int N=1e5+10;
const int M=1e6+10;
const int INF=0x3f3f3f3f;
const int MOD=1e9+7;
int a[N],b[N];
void solve(){
    int n;cin>>n;
    map<int,int> cnt;
    rep(i,0,n) cin>>a[i],cnt[a[i]]++;
    priority_queue<PII,vector<PII>,greater<PII>> pq;
    rep(i,0,n+2) pq.push({cnt[i],i});
    if(a[n-1]>n){
        cout<<-1<<endl;
        return;
    }
    rep(i,0,n){
        PII t=pq.top();pq.pop();
        b[i]=t.y;
        cnt[a[i]]--;
        pq.push({cnt[a[i]],a[i]});
    }
    rep(i,0,n) cout<<b[i]<<' ';
    cout<<endl;
}
int main(){
    IOS;
    //freopen("test.txt", "r", stdin);
    //freopen("test.txt", "w", stdout);
    //int t;cin>>t;
    //while(t--)
        solve();
    return 0;
}