Language: C++

#define debug_interval ','
#define dump_interval ' '
#define debug_toggle 1
//{
#include<bits/stdc++.h>
using namespace std;
#define int long long
#define INT_MAX LLONG_MAX
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define fi first
#define se second
#define mp make_pair
#define rev reverse
#define dans dump(ans)
#define MOD 1000000007
#define amp(v,n) (v).count(n)?v[n]++:v[n]=1;
#define cni cin

//{
inline int toInt(string s){int v;istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x){ostringstream sout;sout<<x;return sout.str();}
template<class...A> inline void dump(){cout<<endl;}
template<class...A> inline void dump_rest() {cout<<endl;}
template<class T, class...A> inline void dump_rest(const T& first, const A&... rest){cout<<dump_interval<<first;dump_rest(rest...);}
template<class T,class...A> inline void dump(const T&first,const A&...rest){cout<<first;dump_rest(rest...);}
template<class...A> inline void debug(){cerr<<endl;}
template<class...A> inline void debug_rest() {cerr<<endl;}
template<class T, class...A> inline void debug_rest(const T& first, const A&... rest){cerr<<debug_interval<<first;debug_rest(rest...);}
template<class T,class...A> inline void debug(const T&first,const A&...rest){if(debug_toggle)cerr<<first,debug_rest(rest...);}
//}

typedef vector<int>vint;
typedef pair<int,int>pint;
typedef vector<pint>vpint;
typedef unsigned int uint;

template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}
template<int m>class mint{private:int i;public:mint() : i(0){}mint(int i): i((i%m+m)%m){}mint operator+(const mint& o){return o.i+i;}mint operator*(const mint& o){return o.i*i;}mint operator-(){return -i;}operator int() {return i;}};



int n,ans;
vint v;
map<int,int> aa;

uint np(int n){
    // nが0以下の時は0とする。
    if (n <= 0) return 0;

    // (n & (n - 1)) == 0 の時は、nが2の冪乗であるため、そのままnを返す。
    if ((n & (n - 1)) == 0) return (uint)n;

    // bitシフトを用いて、2の冪乗を求める。
    uint ret = 1;
    while (n > 0) { ret <<= 1; n >>= 1; }
    return ret;
}

main(){
	cin>>n;
	rep(i,n){int t;cin>>t;v.pb(t);amp(aa,t);};
	sort(rall(v));
	rep(i,v.size()){
		if(aa.count(np(v[i])-v[i])){
			if(aa[v[i]]==1)aa.erase(v[i]);
			else aa[v[i]]--;
			if(aa[np(v[i])-v[i]]==1)aa.erase(np(v[i])-v[i]);
			else aa[np(v[i])-v[i]]--;
			ans++;
		}
	}
	dans;
}


Compiler output:
a.cc:8:9: warning: "INT_MAX" redefined
    8 | #define INT_MAX LLONG_MAX
      |         ^~~~~~~
In file included from /usr/include/c++/14/climits:42,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:38,
                 from a.cc:5:
/usr/lib/gcc/x86_64-linux-gnu/14/include/limits.h:120:9: note: this is the location of the previous definition
  120 | #define INT_MAX __INT_MAX__
      |         ^~~~~~~
a.cc:39:22: error: conflicting declaration 'typedef long long unsigned int uint'
   39 | typedef unsigned int uint;
      |                      ^~~~
In file included from /usr/include/stdlib.h:514,
                 from /usr/include/c++/14/cstdlib:79,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:42:
/usr/include/x86_64-linux-gnu/sys/types.h:150:22: note: previous declaration as 'typedef unsigned int uint'
  150 | typedef unsigned int uint;
      |                      ^~~~
a.cc:64:1: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
   64 | main(){
      | ^~~~