map Codeforces Round #Pi (Div. 2) C. Geometric Progression

题目传送门

 /*
     题意：问选出3个数成等比数列有多少种选法
     map：c1记录是第二个数或第三个数的选法，c2表示所有数字出现的次数。别人的代码很短，思维巧妙
 */
 /************************************************
  * Author        :Running_Time
  * Created Time  :2015-8-6 1:07:18
  * File Name     :C.cpp
  ************************************************/

 #include <cstdio>
 #include <algorithm>
 #include <iostream>
 #include <sstream>
 #include <cstring>
 #include <cmath>
 #include <string>
 #include <vector>
 #include <queue>
 #include <deque>
 #include <stack>
 #include <list>
 #include <map>
 #include <set>
 #include <bitset>
 #include <cstdlib>
 #include <ctime>
 using namespace std;

 #define lson l, mid, rt << 1
 #define rson mid + 1, r, rt << 1 | 1
 typedef long long ll;
 const int MAXN = 2e5 + ;
 const int INF = 0x3f3f3f3f;
 const int MOD = 1e9 + ;
 map<ll, ll> c1, c2;

 int main(void)    {     //Codeforces Round #Pi (Div. 2) C. Geometric Progression
     ll ans = , x;  ll n, k;
     scanf ("%I64d%I64d", &n, &k);
     for (int i=; i<=n; ++i)    {
         scanf ("%I64d", &x);
         if (x % (k * k) == )   ans += c1[x/k];     //x可选作第三个数
         if (x % k == ) c1[x] += c2[x/k];           //x第三个数或第二个数
         c2[x]++;
     }

     printf ("%I64d\n", ans);

     return ;
 }