1819A - Constructive Problem - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;


ll mex1(ll a[],ll n,set<ll>s)
{
    ll mex;
    for(ll i=0;i<=1000000000;i++)
    {
        if(s.count(i)==0)
        {
            mex=i;
            break;
        }
    }
    return mex;
}

int main()
{
    cin.tie(0) -> sync_with_stdio(0);
    int T;
    cin>>T;
    while(T--)
    {
        ll n,mex,f=0;
        cin>>n;
        ll a[n],max=-1;
        set<ll> s1,s2;
        map<ll,ll> first,last,count;
        for(ll i=0;i<n;i++)
        {
            cin>>a[i];
            s1.insert(a[i]);
            if(first[a[i]]==0) first[a[i]]=i+1;
            last[a[i]]=i+1;
            count[a[i]]++;
            if(a[i]>max) max=a[i];
        }
        mex=mex1(a,n,s1);
        if(mex==0) cout<<"YES\n";
        else if(s1.count(mex+1)==0)
        {
            ll q=0;
            for(ll i=0;i<mex;i++)
            {
                if(count[i]>1)
                {
                    q++;
                    break;
                }
            }
            if(max>mex) q++;
            if(q==0) cout<<"NO\n";
            else cout<<"YES\n";
        }
        else
        {
            //cout<<first[mex+1]<<" "<<last[mex+1]<<"\n";
            for(ll i=0;i<n;i++)
            {
                if(i>=first[mex+1]-1 && i<=last[mex+1]-1) a[i]=mex;
                s2.insert(a[i]);
            }
            if(mex1(a,n,s2)==mex+1) cout<<"YES\n";
            else cout<<"NO\n";
        }

    }
    return 0;
}