Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 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 area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)


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

// pp basics

#define in ,
#define esc_paren(...) __VA_ARGS__
#define pp_empty(...)
#define pp_cat_i(x, y) x ## y
#define pp_cat(x, y) pp_cat_i(x, y)

#define pp_inc0 1
#define pp_inc1 2
#define pp_inc2 3
#define pp_inc3 4
#define pp_inc4 5
#define pp_inc5 6
#define pp_inc6 7
#define pp_inc7 8
#define pp_inc8 9
#define pp_inc(i) pp_cat(pp_inc, i)

// pp args counter

#define pp_arg10(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, ...) _9
#define pp_has_comma(...) pp_arg10(__VA_ARGS__, 1, 1, 1, 1, 1, 1, 1, 1, 0)
#define pp_trigger_paren(...) ,
#define pp_is_empty(...) \
  pp_is_empty1( \
      pp_has_comma(__VA_ARGS__), \
      pp_has_comma(pp_trigger_paren __VA_ARGS__), \
      pp_has_comma(__VA_ARGS__()), \
      pp_has_comma(pp_trigger_paren __VA_ARGS__()) \
      ) 
#define pp_paste5(_0, _1, _2, _3, _4) _0 ## _1 ## _2 ## _3 ## _4
#define pp_is_empty1(_0, _1, _2, _3) pp_has_comma(pp_paste5(pp_is_empty_case_, _0, _1, _2, _3))
#define pp_is_empty_case_0001 ,

#define pp_is_one(...) pp_is_one_(__VA_ARGS__, pp_one_seq)
#define pp_is_one_(...) pp_is_one_n(__VA_ARGS__)
#define pp_is_one_n(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10, N, ...) N
#define pp_one_seq 0,0,0,0,0,0,0,0,0,1,0

#define pp_narg(...) \
         pp_narg_(__VA_ARGS__, pp_narg_seq)
#define pp_narg_(...) \
         pp_arg_n(__VA_ARGS__)
#define pp_narg_n( \
          _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
         _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
         _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
         _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
         _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
         _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
         _61,_62,_63,n,...) n
#define pp_narg_seq \
         63,62,61,60, \
         59,58,57,56,55,54,53,52,51,50, \
         49,48,47,46,45,44,43,42,41,40, \
         39,38,37,36,35,34,33,32,31,30, \
         29,28,27,26,25,24,23,22,21,20, \
         19,18,17,16,15,14,13,12,11,10, \
          9, 8, 7, 6, 5, 4, 3, 2, 1, 0

// pp if
#define pp_if_0(x, y) y
#define pp_if_1(x, y) x
#define pp_if(cond, x, y) pp_cat(pp_if_, cond)(x, y)

// pp foreach
#define pp_foreach_i9(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i8(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i7(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i6(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i5(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i4(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i3(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i2(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i1(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i0(i, f, x, ...) f(x) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach_i(i, f, ...) pp_if(pp_is_empty(__VA_ARGS__), pp_empty, pp_cat(pp_foreach_i, i))(pp_inc(i), f, __VA_ARGS__)
#define pp_foreach(f, ...) pp_foreach_i(0, f, __VA_ARGS__)

// alternative rep macro
#define rep(...) rep1(rep2, (__VA_ARGS__))
#define rep1(X, A) X A
#define rep2(i, ...) pp_if(pp_is_one(__VA_ARGS__), rep_len1(i, __VA_ARGS__), rep3(i, __VA_ARGS__))
#define rep_len1(i, end, ...) for (int i = 0; i < end; i++)
#define rep3(i, iter, ...) rep4(rep5, (iter, i, esc_paren __VA_ARGS__))
#define rep4(X, A) X A
#define rep5(iter, ...) iter(__VA_ARGS__)

// iterators
#define up up_iter ,
#define up_iter3(i, start, end) for (int i = start; i < end; i++)
#define up_iter4(i, start, end, up) for (int i = start; i < end; i += up)
#define up_iter(...) CAT(up_iter, PP_NARG(__VA_ARGS__))(__VA_ARGS__)
#define down down_iter ,
#define down_iter2(i, end) for(int i = end-1; i >= 0; i++)
#define down_iter3(i, start, end) for (int i = end-1; i >= start; i--)
#define down_iter4(i, start, end, down) for (int i = end-1; i >= start; i -= down)
#define down_iter(...) CAT(down_iter, PP_NARG(__VA_ARGS__))(__VA_ARGS__)
#define viter viter_iter ,
#define viter_iter(i, v) auto tmpitr=v.begin(); for (auto i=*tmpitr; tmpitr != v.end(); tmpitr++)

// debug macros
#define debug_var_elem(v) << #v << "=" << v << " "
#ifdef DEBUG
#define debug_echo(e) cout << "L" << __LINE__ << ": " << e << endl
#define debug_var(...) cout << "L" << __LINE__ << ": " pp_foreach(debug_var_elem, __VA_ARGS__) << endl
#else
#define debug_echo(e)
#define debug_var(...)
#endif

// short names
typedef long long ll;
#define down_queue(x) priority_queue<x>
#define up_queue(x) priority_queue<x, vector<x>, greater<x>>
ll gcd(ll a,ll b){while(b){ll tmp = a % b;a = b;b = tmp;}return a;}
ll lcm(ll a,ll b){return a / gcd(a,b) * b;}
#define all(x) x.begin(), x.end()
#define split_str(str, sp_word) istringstream stream(str); string res; for(int cnt = 0; getline(stream,res,sp_word); cnt++)
#define digit(x) ((int)log10((double)(x)) + 1)
#define mp(x,y) make_pair(x,y)
#define pb(x) push_back(x)
#define pf(x) push_front(x)
#define vec_cpy(to,from) copy(all(to),back_inserter(from))
#define ary_cpy(to,from) memcpy(to, from, sizeof(from))
#define MOD 1000000007 

// pp unionfind

#define pp_gen_field(list) pp_gen_field1 list
#define pp_gen_field1(name, val) decltype(val) name;
#define pp_gen_init(list) pp_gen_init1 list
#define pp_gen_init1(name, val) ,val
#define pp_gen_find_set_wrap(ufnodename) ufnodename pp_gen_find_set
#define pp_gen_find_set(list) pp_gen_find_set1 list
#define pp_gen_find_set1(name, val) \
  find_set_ ## name(int x, decltype(val) v) { \
    if (nodes[x].parent == x) { \
      nodes[x].name = v; \
      return nodes[x]; \
    } else { \
      return nodes[x] = find_set_ ## name(nodes[x].parent, v); \
    } \
  }
#define pp_gen_unite_set(list) pp_gen_unite_set1 list
#define pp_gen_unite_set1(name, val) \
  if (nodes[x].name == val) { \
    find_set_ ## name(x, find(y).name); \
  } \
  if (nodes[y].name == val) { \
    find_set_ ## name(y, find(x).name); \
  }

#define def_unionfind(ufname, ufnodename, ...) \
  struct ufnodename { \
    int parent; \
    pp_foreach(pp_gen_field, __VA_ARGS__) \
  }; \
  struct ufname { \
    vector<ufnodename> nodes; \
    vector<int> rank; \
    ufname(int n) : nodes(n+1), rank(n+1) { \
      rep (i in n) { \
        nodes[i] = ufnodename{i pp_foreach(pp_gen_init, __VA_ARGS__)}; \
        rank[i] = 0; \
      } \
    } \
    ufnodename find(int x) { \
      if (nodes[x].parent == x) { \
        return nodes[x]; \
      } else { \
        return nodes[x] = find(nodes[x].parent); \
      } \
    } \
    pp_foreach(pp_gen_find_set_wrap(ufnodename), __VA_ARGS__) \
    void unite(int x, int y) { \
      x = find(x).parent; \
      y = find(y).parent; \
      if (x == y) return; \
      if (rank[x] < rank[y]) { \
        pp_foreach(pp_gen_unite_set, __VA_ARGS__); \
        nodes[x].parent = y; \
      } else { \
        pp_foreach(pp_gen_unite_set, __VA_ARGS__); \
        nodes[y].parent = x; \
        if (rank[x] == rank[y]) rank[x]++; \
      } \
    } \
    bool same(int x, int y) { \
      return find(x).parent == find(y).parent; \
    } \
  }

template <typename T, typename K>
bool has_key(T cont, K key) {
  auto it = cont.find(key);
  return it != cont.end();
}

//
// Implementation
//

#define MAX_N 100000
int N;
int A[MAX_N];
unordered_map<int, int> acnt;

int main() {
  cin >> N;
  rep (i in N) cin >> A[i];
  rep (i in N) {
    if (!has_key(acnt, A[i])) {
      acnt[A[i]] = 0;
    }
    acnt[A[i]]++;
  }
  vector<int> bars;
  for (auto e : acnt) {
    debug_var(e.first, e.second);
    if (e.second >= 4) {
      bars.pb(e.first);
      bars.pb(e.first);
    } else if (e.second >= 2) {
      bars.pb(e.first);
    }
  }
  sort(all(bars));
  if (bars.size() < 2) {
    cout << 0 << endl;
  } else {
    cout << bars[bars.size()-1] * bars[bars.size()-2] << endl;
  }
}