time limit per test : 1 second memory limit per test : 256 megabytes input : standard input output : standard output
Problem Description
Polycarp loves geometric progressions very much. Since he was only three years old, he loves only the progressions of length three. He also has a favorite integer (k) and a sequence (a), consisting of (n) integers.
He wants to know how many subsequences of length three can be selected from (a), so that they form a geometric progression with common ratio (k).
A subsequence of length three is a combination of three such indexes (i_1, i_2, i_3), that (1 ≤ i_1 < i_2 < i_3 ≤ n). That is, a subsequence of length three are such groups of three elements that are not necessarily consecutive in the sequence, but their indexes are strictly increasing.
A geometric progression with common ratio (k) is a sequence of numbers of the form (b·k^0, b·k^1, ..., b·k^{r - 1}).Polycarp is only three years old, so he can not calculate this number himself. Help him to do it.
Input
The first line of the input contains two integers, (n) and (k (1 ≤ n, k ≤ 2·10^5)), showing how many numbers Polycarp's sequence has and his favorite number.
The second line contains (n) integers (a_1, a_2, ..., a_n ( - 10^9 ≤ a_i ≤ 10^9)) — elements of the sequence.
Output
Output a single number — the number of ways to choose a subsequence of length three, such that it forms a geometric progression with a common ratio (k).
Examples
input
5 2
1 1 2 2 4
output
4
input
3 1
1 1 1
output
1
input
10 3
1 2 6 2 3 6 9 18 3 9
output
6
Note
In the first sample test the answer is four, as any of the two 1s can be chosen as the first element, the second element can be any of the 2s, and the third element of the subsequence must be equal to 4.
题意
在(n)个数中选出三个数，要求组成的序列公比为(k)，并且不改变这三个数的前后关系，问有多少种选择方案
思路
(DP)，因为(- 10^9 ≤ a_i ≤ 10^9) ，所以还需要用到(map)
定义(dp[i][j])表示把数字(j)放在等比数列第(i)个位置的方案数
(dp[i][j]+=dp[i-1][j/k])
将所有长度为(3)的方案数加起来即可
代码
#include <bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define ms(a,b) memset(a,b,sizeof(a))
const int inf=0x3f3f3f3f;
const ll INF=0x3f3f3f3f3f3f3f3f;
const int maxn=1e6+10;
const int mod=1e9+7;
const int maxm=1e3+10;
using namespace std;
ll a[maxn];
int main(int argc, char const *argv[])
{
#ifndef ONLINE_JUDGE
freopen("/home/wzy/in", "r", stdin);
freopen("/home/wzy/out", "w", stdout);
srand((unsigned int)time(NULL));
#endif
ios::sync_with_stdio(false);
cin.tie(0);
int n,k;
cin>>n>>k;
map<int,ll>dp[4];
ll ans=0;
for(int i=1;i<=n;i++)
{
cin>>a[i];
if(a[i]%k==0)
{
ans+=dp[2][a[i]/k];
dp[2][a[i]]+=dp[1][a[i]/k];
}
dp[1][a[i]]++;
}
cout<<ans<<endl;
#ifndef ONLINE_JUDGE
cerr<<"Time elapsed: "<<1.0*clock()/CLOCKS_PER_SEC<<" s."<<endl;
#endif
return 0;
}