Problem:
Score : 600 points

Problem Statement

Takahashi has N balls with positive integers written on them. The integer written on the i-th ball is A_i.
He would like to form some number of pairs such that the sum of the integers written on each pair of balls is a power of 2.
Note that a ball cannot belong to multiple pairs.
Find the maximum possible number of pairs that can be formed.

Here, a positive integer is said to be a power of 2 when it can be written as 2^t using some non-negative integer t.

Constraints

1 \leq N \leq 2\times 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible number of pairs such that the sum of the integers written on each pair of balls is a power of 2.

Sample Input 1

3
1 2 3

Sample Output 1

1

We can form one pair whose sum of the written numbers is 4 by pairing the first and third balls.
Note that we cannot pair the second ball with itself.

Sample Input 2

5
3 11 14 5 13

Sample Output 2

2

Solution in OCaml (4.02.3) (judge verdict: Wrong Answer)

open Printf open Scanf
module Lib = struct
  let i32,i64,ist = Int64.(to_int,of_int,to_string)
  let (+$) = (+) let (-$) = (-) let (/$) = (/) let ( *$) = ( * ) let (%$) = (mod)
  let labs = Int64.abs
  let (+),(-),( * ),(/),(mod),(%) = Int64.(add,sub,mul,div,rem,rem)
  let (+=) r v = r := !r + v
  let (-=) r v = r := !r - v
  let ( *= ) r v = r := !r * v
  let (/=) r v = r := !r / v
  let (%=) r v = r := !r % v
  let max_i64,min_i64 = Int64.(max_int,min_int)
  (* math *)
  let ceildiv p q = (p+q-1L) / q
  let rec gcd m n = match m,n with | m,0L -> m | m,n -> gcd n (m mod n)
  let lcm m n = m / gcd m n * n
  module Int64_infix = struct
    let (land),(lor),(lxor),lnot,(lsl),(lsr),(asr) =
      let open Int64 in
      (logand),(logor),(logxor),lognot,
      (fun u v -> shift_left u (i32 v)),
      (fun u v -> shift_right u (i32 v)),
      (fun u v -> shift_right_logical u (i32 v))
  end
  let pow n k =
    let rec f a n = function
    | 0L -> a | 1L -> n*a | k when k mod 2L = 0L -> f a (n*n) (k/2L) | k -> f (a*n) (n*n) (k/2L)
    in f 1L n k
  (* input *)
  let input_i64_array n = Array.init (i32 n) (fun _ -> Scanf.scanf " %Ld" (fun v -> v)) let iia = input_i64_array
  let get_i64 _ = Scanf.scanf " %Ld" (fun v -> v) let gi = get_i64
  let get_2_i64 _ = Scanf.scanf " %Ld %Ld" (fun u v -> u,v) let g2 = get_2_i64
  let get_3_i64 _ = Scanf.scanf " %Ld %Ld %Ld" (fun u v w -> u,v,w) let g3 = get_3_i64
  let get_4_i64 _ = Scanf.scanf " %Ld %Ld %Ld %Ld" (fun u v w x -> u,v,w,x) let g4 = get_4_i64
  let get_5_i64 _ = Scanf.scanf " %Ld %Ld %Ld %Ld %Ld" (fun u v w x y -> u,v,w,x,y) let g5 = get_5_i64
  let i32_get_int _ = Scanf.scanf " %d" (fun v -> v) let i32_get_2_ints _ = Scanf.scanf " %d %d" (fun u v -> u,v) let i32_get_3_ints _ = Scanf.scanf " %d %d %d" (fun u v w -> u,v,w) let i32_get_4_ints _ = Scanf.scanf " %d %d %d %d" (fun u v w x -> u,v,w,x) let i32_get_5_ints _ = Scanf.scanf " %d %d %d %d %d" (fun u v w x y -> u,v,w,x,y)
  (* utils *)
  external id : 'a -> 'a = "%identity"
  let ( *< ) f g x = f (g x)
  let ( *> ) f g x = g (f x)
  let rv f p q = f q p (* ex. 24 |> rv (/) 6 *)
  let alen a = i64 @@ Array.length a
  let llen l = i64 @@  List.length l
  let slen s = String.length s
  let string_of_list ?(separator=" ") f ls = ls |> List.map f |> String.concat separator
  let string_of_array ?(separator=" ") f a = a |> Array.to_list |> string_of_list ~separator f
  let char_list_of_string str = let rec f0 i a = if i<0 then a else f0 (i-$1) (str.[i]::a) in f0 (String.length str -$ 1) []
  let string_of_char_list ls = List.fold_left (fun u v -> u ^ (String.make 1 v)) "" ls
  let fail _ = failwith "fail"
  let dx,dy = [|1;0;-1;0|],[|0;1;0;-1|]
  let range l r = let ls = ref [] in for i=i32 l to i32 r do ls := i :: !ls done;!ls |> List.rev
  let cartesian_product u v = u |> List.map (fun p -> List.map (fun q -> p,q) v) |> List.flatten
  let shuffle a = Array.sort (fun _ _ -> (Random.int 3) -$ 1) a
  let binary_search ng ok f = let d = if ng < ok then 1L else -1L in
    let rec f0 ng ok =
      if labs (ok - ng) <= 1L then ok
      else let mid = ng + (ok - ng) / 2L in if f mid then f0 ng mid else f0 mid ok
    in f0 (ng-1L*d) (ok+1L*d)
  let lower_bound a v = binary_search 0L (alen a - 1L) (fun i -> a.(i32 i) >= v)
  let upper_bound a v = binary_search 0L (alen a - 1L) (fun i -> a.(i32 i) >  v)
  let equal_range a v = lower_bound a v, upper_bound a v (* #=snd-fst *)
  let rec fix f x = f (fix f) x
  let fix_memo ?(size=10000) f x =
    let tb = Hashtbl.create ~random:true size in
    let rec f0 x =
      try Hashtbl.find tb x with
      | Not_found -> (let v = f f0 x in Hashtbl.add tb x v; v)
    in f0 x
  (* imperative *)
  let rep f t ?(stride=1L) fbod = let i = ref f in while !i <= t do fbod !i; i := !i + stride done
  let repb f t ?(stride=1L) fbod = let i,c = ref f,ref true in while !i <= t && !c do
    match fbod !i with | `Break -> c := false | `Ok -> i := !i + stride done
  let repm f t ?(stride=1L) m0 fbod =
    let i,c,m = ref f,ref true,ref m0 in
    while !i<=t && !c do match fbod !m !i with
    | `Break -> c := false | `Break_m m' -> (c := false; m := m') | `Ok m' -> m := m'; i := !i + stride done; !m
  let repi f t ?(stride=1) fbod = let i = ref f in while !i <= t do fbod !i; i := !i +$ stride done
  let repib f t ?(stride=1) fbod = let i,c = ref f,ref true in while !i <= t && !c do
    match fbod !i with | `Break -> c := false | `Ok -> i := !i +$ stride done
  let repim f t ?(stride=1) m0 fbod =
    let i,c,m = ref f,ref true,ref m0 in
    while !i<=t && !c do match fbod !m !i with
    | `Break -> c := false | `Break_m m' -> (c := false; m := m') | `Ok m' -> m := m'; i := !i +$ stride done; !m
  let rep_rev f t ?(stride=1L) fbod = rep t f ~stride @@ fun v -> fbod @@ f + t - v
  let repb_rev f t ?(stride=1L) fbod = repb t f ~stride @@ fun v -> fbod @@ f + t - v
  let repm_rev f t ?(stride=1L) m0 fbod = repm f t ~stride m0 @@ fun u v -> fbod u @@ f + t - v
  let repi_rev f t ?(stride=1) fbod = repi t f ~stride @@ fun v -> fbod @@ f +$ t -$ v
  let repib_rev f t ?(stride=1) fbod = repib t f ~stride @@ fun v -> fbod @@ f +$ t -$ v
  let repim_rev f t ?(stride=1) m0 fbod = repim f t ~stride m0 @@ fun u v -> fbod u @@ f +$ t -$ v
  (* output *)
  let print_list_mle f ls = string_of_list f ls |> print_endline
  let print_array_mle f a = string_of_array f a |> print_endline
  let print_list f ls = ls |> List.iter (fun v -> printf "%s " @@ f v); print_newline ()
  let print_array f a = a |> Array.iter (fun v -> printf "%s " @@ f v); print_newline ()
  (* debug *)
  let dump_2darr f a = a |> Array.iter (fun b -> print_array f b)
  let dump_i64_list ls = print_list ist ls
  let dump_i64_array a = print_array ist a
end open Lib open Array

module IMap = Map.Make(Int64)

module UnionFind_i32 = struct
  let init n = Array.make (i32 n) (-1)
  let is_root i a = a.(i) < 0
  let rec root_destructive x a = (* find the root of the tree x belongs to *)
    if a.(x) < 0 then x
    else (
      a.(x) <- root_destructive a.(x) a;
      a.(x))
  let size_destructive i a = let r = root_destructive i a in -r
  let same_destructive x y a = root_destructive x a = root_destructive y a
  let connect_destructive x y a = (* merge the trees x and y belongs to respectively *)
    let p,q = root_destructive x a,root_destructive y a
    in if p = q then () else (
      let p,q = if a.(p) > a.(q) then q,p else p,q
      in
      a.(p) <- a.(p) +$ a.(q);
      a.(q) <- p)
end
let coordinate_compression a =
  (* zip:i64->int, unzip:int->i64 *)
  let ls = a |> Array.to_list |> List.sort_uniq compare in
  let unzip = Array.make (List.length ls) 0L in
  let zip = fst @@ List.fold_left (fun (m,i) v ->
    unzip.(i) <- v; IMap.add v i m,i+$1) (IMap.empty,0) ls
  in zip,unzip
let () =
  let n=gi 0 in let a = iia n in
  let mp = ref IMap.empty in
  iter (fun v ->
    mp := IMap.add v (1L + try IMap.find v !mp with _ -> 0L) !mp
  ) a;
  let zip,unz = coordinate_compression a in
  let uf = UnionFind_i32.init (alen unz) in
  let size = init (i32 @@ alen unz) (fun i ->
    let v = unz.(i) in
    let z = fix (fun f a -> if a>v then a else f (a*2L)) 1L in
    (* printf "%Ld %Ld\n" z v; *)
    if z=v+v then IMap.find v !mp else 1L)
  in
  let size_con x y a = UnionFind_i32.(
    let p,q = root_destructive x a,root_destructive y a
    in if p = q then () else (
      let p,q = if a.(p) > a.(q) then q,p else p,q
      in
      size.(p) <- size.(p) + size.(q);
      size.(q) <- 0L
    )
  ) in
  IMap.iter (fun v c ->
    let z = fix (fun f a -> if a>v then a else f (a*2L)) 1L in
    let cp = z - v in
    (* printf "%Ld + %Ld = %Ld\n" v cp z; *)
    try
      let _ = IMap.find cp !mp in
      let p,q = IMap.find v zip,IMap.find cp zip in
      size_con p q uf;
      UnionFind_i32.connect_destructive p q uf;
      (* print_array (sprintf "%2d") uf;
      print_array (sprintf "%2Ld") size; *)
    with _ -> ()
  ) !mp;
  let r = ref 0L in iter (fun v ->
    if v>=2L then r += v / 2L
  ) size; printf "%Ld" @@ !r