1697 - Chess Tournament.cpp

/*
Problem Name: Chess Tournament
Problem Link: https://cses.fi/problemset/task/1697
Author: Sachin Srivastava (mrsac7)
*/
#include<bits/stdc++.h>
using namespace std;

#define int long long
#define endl '\n'

signed main(){
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    #ifdef LOCAL
    freopen("input.txt", "r" , stdin);
    freopen("output.txt", "w", stdout);
    #endif
    
    int n; cin>>n;
    int a[n+1];
    set<pair<int,int>> st;
    int sm = 0;
    for (int i = 1; i <= n; i++) {
        cin>>a[i];
        sm += a[i];
        if (a[i])
            st.insert({a[i], i});
    }
    sm;
    vector<pair<int,int>> ans;
    while(st.size()>=2) {
        auto [x1, i1] = *st.begin(); st.erase({x1, i1});
        vector<pair<int,int>> s;
        while(x1>0 && st.size()) {
            auto [x2, i2] = *st.rbegin(); st.erase({x2, i2});    
            ans.push_back({i1, i2});
            x1--, x2--;
            s.push_back({x2, i2});
        }
        for (auto [i, j]: s) {
            if (i>0) st.insert({i, j});
        }
    }
    if (2*ans.size() == sm) {
        cout<<ans.size()<<endl;
        for (auto [i, j]: ans) {
            cout<<i<<' '<<j<<endl;
        }
    }
    else
        cout<<"IMPOSSIBLE";
}