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

题目传送门

 1 /*
 2     题意：问选出3个数成等比数列有多少种选法
 3     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
 4 */
 5 /************************************************
 6  * Author        :Running_Time
 7  * Created Time  :2015-8-6 1:07:18
 8  * File Name     :C.cpp
 9  ************************************************/
10
11 #include <cstdio>
12 #include <algorithm>
13 #include <iostream>
14 #include <sstream>
15 #include <cstring>
16 #include <cmath>
17 #include <string>
18 #include <vector>
19 #include <queue>
20 #include <deque>
21 #include <stack>
22 #include <list>
23 #include <map>
24 #include <set>
25 #include <bitset>
26 #include <cstdlib>
27 #include <ctime>
28 using namespace std;
29
30 #define lson l, mid, rt << 1
31 #define rson mid + 1, r, rt << 1 | 1
32 typedef long long ll;
33 const int MAXN = 2e5 + 10;
34 const int INF = 0x3f3f3f3f;
35 const int MOD = 1e9 + 7;
36 map<ll, ll> c1, c2;
37
38 int main(void)    {     //Codeforces Round #Pi (Div. 2) C. Geometric Progression
39     ll ans = 0, x;  ll n, k;
40     scanf ("%I64d%I64d", &n, &k);
41     for (int i=1; i<=n; ++i)    {
42         scanf ("%I64d", &x);
43         if (x % (k * k) == 0)   ans += c1[x/k];     //x可选作第三个数
44         if (x % k == 0) c1[x] += c2[x/k];           //x第三个数或第二个数
45         c2[x]++;
46     }
47
48     printf ("%I64d\n", ans);
49
50     return 0;
51 }