995D - Game - CodeForces Solution

// #pragma GCC optimize("Ofast,unroll-loops")
// #pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2,fma,tune=native")

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

#include <bits/extc++.h>
using namespace __gnu_pbds;

template<class T>
using ordered_set = tree< T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

template<class T>
using ordered_multiset = tree< T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;


#define ll long long
#define ull unsigned long long
#define ld long double
#define pb push_back
#define eb emplace_back
#define setpre(i) setprecision(i)<<fixed
#define foru(i, a, b) for(int i=a;i<=b;++i)
#define ford(i, a, b) for(int i=a;i>=b;--i)
#define mp make_pair
#define moo cout<<"moo "
#define fi first
#define se second

ll lmod = 1e9 + 7;
typedef vector<int> vi;
typedef vector< vector<int> > vvi;
typedef pair<int, int> ii;
typedef pair<double, double> dd;

ll arr[(1 << 18)];

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    int n, r; cin >> n >> r;
    ll sum = 0;
    foru(i, 0, (1 << n) - 1){
        cin >> arr[i]; sum += arr[i];
    }
    cout << setpre(6);
    cout << (double)sum / (double)(1 << n) << endl;
    while(r--){
        int x, y; cin >> x >> y;
        sum -= arr[x];
        arr[x] = y;
        sum += arr[x];
        cout << (double)sum / (double)(1 << n) << endl;
    }

    return 0;
}