702 B. Powers of Two.cpp

/*
problem link: https://codeforces.com/contest/702/problem/B
my solution on codeforces : https://codeforces.com/contest/651/submission/174600594
problem name: B. Powers of Two
status: accepted
author: lilian_steven   or LELLO 
*/
#include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <cmath>
#include <unordered_map>
#include <algorithm>
#include <iomanip>
#include <bitset>
#include <utility>
#include <string>
#include <cctype>
#include <iomanip>
#include <numeric> // for NTH
#define bye return;
#define ll long long
#define lli long long int
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) ((x).size())
#define ison(num, bit) (num & (1 << bit))
#define setoff(num, bit) (num ^= (1 << bit)) // iff (num &(1<<bit))
#define flipbit(num, bit) (num ^= (1 << bit))
#define setbit(num, bit) (num |= (1 << bit))
#define clearbit(num, bit) (num &= ~(1 << bit))
#define pop_count(num) __builtin_popcount(num)
#define Num_of_Digits(n) ((ll)log10(n) + 1)
#define ceill(n, m) (((n) / (m)) + ((n) % (m) ? 1 : 0))
#define lastOne(x) (__builtin_clzll(x))
#define Pi  3.14159265358979323846264
#define sin(a) sin((a)*PI/180)
#define cos(a) cos((a)*PI/180)
ll sgn(ll x){ if(x > 0) return 1; if(x < 0) return -1; return 0;}
ll gcd(ll a, ll b){if(b == 0) return a; return gcd(b, a%b);}
ll lcm(ll a, ll b){return a/gcd(a, b)*b;}
using namespace std;
void AC() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr); // or cin.tie(NULL)
    cout.tie(nullptr);  // or cout.tie(0)
    // #ifndef ONLINE_JUDGE
    //     freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
    // #endif
}
bool cmp(pair<int,int>&a, pair<int,int>&b){
    if(a.first == b.first){
        return a.second > b.second;
    }
    return a.first < b.first;
}
// int dx[] = {1,-1,0,0} , dy[] = {0,0,1,-1};  // 4 Direction
// int dx[] = {1,-1,0,0,1,1,-1,-1}  , dy[] = {0,0,1,-1,1,-1,1,-1};  // 8 Direction
// int dx[] = {1,-1,1,-1,2,2,-2,-2} ,  dy[] = {2,2,-2,-2,1,-1,1,-1}; // Knight Direction
// int dx[] = {2,-2,1,1,-1,-1} ,  dy[] = {0,0,1,-1,1,-1}; // Hexagonal Direction

void LELLO(){
    //author: lilian_steven   or LELLO 
    ll n , ans(0) ; cin >> n;
    vector<int> vc(n);
    map<ll , ll> mp ; 
    for (int i = 0; i < n; i++)
    {
        cin >> vc[i];
    }
    sort(all(vc));
    ll z = *max_element(all(vc))*2 ;
    for (int i = 0; i < n; i++)
    {
        for (int p = 1; p <= z; p*=2)
        {
            ans += mp[p-vc[i]] ;
        }
        mp[vc[i]]++ ;
    }
    cout<< ans <<endl; 
}

void driver(ll t=1)
{
    while (t--) LELLO();
}
signed main()
{
    AC();
    // ll test ; cin >> test ;
    // chrono::time_point<chrono::system_clock> start, end;
    // start = chrono::system_clock::now();
    driver();
    // end = chrono::system_clock::now();
    // chrono::duration<double> elapsed_seconds = end - start;
    // time_t end_time = chrono::system_clock::to_time_t(end);
    // cout << "finished computation at " << ctime(&end_time) << "elapsed time: " << elapsed_seconds.count() << "s\n";
    // cerr << "time taken : " << (float)clock() / CLOCKS_PER_SEC << " secs" << endl ;

}