cf976d Degree Set

ref

#include <algorithm>
#include <iostream>
#include <cstdio>
#include <vector>
using namespace std;
int n, d[305], deg[1005];
vector<pair<int,int> > vec;
int main(){
	cin>>n;
	for(int i=1; i<=n; i++)
		scanf("%d", &d[i]);
	int l=1, r=n, q=d[n]+1, lst=0;
	for(int i=1; i<=q; i++){
		if(l<=r && q-i+deg[i]==d[r]){
			lst = 1;
			r--;
		}
		else if(l<=r && deg[i]==d[l]){
			lst = 0;
			l++;
		}
		if(lst){
			for(int j=i+1; j<=q; j++){
				deg[i]++; deg[j]++;
				vec.push_back(make_pair(i,j));
			}
		}
	}
	cout<<vec.size()<<endl;
	for(int i=0; i<vec.size(); i++)
		printf("%d %d\n", vec[i].first, vec[i].second);
	return 0;
}