404C - Restore Graph - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n'
#define output(x) cout << x << endl
#define print(x) cout << #x << " = " << x << endl
#define yes cout<<"YES\n"
#define no cout<<"NO\n"
#define pi acos(-1)
 
 
int32_t main()
{
 
    ios::sync_with_stdio(false);
    cin.tie(0);
    int t=1,cas=1;
    int n,k;

 	while(t--)
 	{
 		int n,k,i,j,f=0,mx=0;
 		cin>>n>>k;
 		int a[n+5];
 		vector<int>v[n+5];
 		vector<pair<int,int>>adj;
 		for(i=1;i<=n;i++)
 		{
 			cin>>a[i];
 			mx=max(mx,a[i]);
 			v[a[i]].push_back(i);
 		}
 		if(v[0].size()!=1)
 		{
 			cout<<-1<<endl;
 			continue;
 		}
 		for(i=1;i<=mx;i++)
 		{
 			if(v[i-1].size()*k<v[i].size())
 			{
 				f=1;break;
 			}
 		}
 		if(f)
 		{
 			cout<<-1<<endl;
 			continue;
 		}
 		int cnt1=0,g=0;
 		for(i=1;i<=mx;i++)
 		{
 			int l=0,cnt=0,ff=0,ggg=0;cnt1=0;
 			for(j=0;j<v[i-1].size();)
 			{
 				for(;l<v[i].size();l++)
 				{
 					adj.push_back({v[i-1][j],v[i][l]});
 					cnt++;cnt1++;
 					int last= k;
 					if(i-1!=0) last=k-1; 
 					if(cnt>last)
 					{
 						ggg=1;break;
 					}
 					if(cnt==last) 
 					{
 						cnt=0;
 						j++;l++;break;
 						
 					}
 				}//cout<<"for "<<i<<" com "<<cnt1<<" sz "<<v[i].size()<<endl;
 				if(ggg) break;
 				if(cnt1>=v[i].size()) {ff=1;break;} 
 			}if(!ff) {
 				g=1;break;
 			}
 		}
 		if(g) 
 		{
 			cout<<-1<<endl;
 			continue;
 		}
 		cout<<adj.size()<<endl;
 		for(auto [x,y]:adj)
 			cout<<x<<' '<<y<<endl;
 		
 	}
    
 
    return 0;
}