【DP】【数论】17.6.3 计算器 题解

P<= 100，所以x的所有质因子都小于P，所以可以先预处理出[1,1e9]的质因数（不到3e6个），在从小到大枚举，DP一遍

#include <iostream>#include <cstdio>#include <cstdlib>#include <algorithm>#include <cstring>#include <cmath>#include <string>using namespace std;typedef long long LL;const int Maxn=3000005,inf=999999999+208;int l,r,p;int pr[30],pn;int num[Maxn],f[Maxn],n;bool flag[Maxn];inline bool isprime(int x) {    for(int i = 2; i*i <= x; i++) if(!(x%i)) return false;    return true;}void dfs(int idx, LL v) {    if(idx > pn) num[++n] = v, f[n] = inf;    else for(; v <= r; v *= pr[idx]) dfs(idx+1, v);}int main() {    freopen("calculator.in","r",stdin);    freopen("calculator.out","w",stdout);    cin>>l>>r>>p;    for(int i = 2; i <= p; i++) if(isprime(i)) pr[++pn] = i;    dfs(1, 1); sort(num+1, num+n+1);    f[1] = 0; flag[1] = true;    for (int i = 2, N = num[n]; i < p; i++) {        for(int j = 1, w = 0; num[j]*i <= N; j++) if(f[j]+i < p) {            while(num[++w] < num[j]*i);            f[w] = min(f[w], f[j]+1);            flag[w] = true;        }    }    int ans = 0;    for(int i = 1; i <= n; i++) if(num[i] >= l && flag[i]) ++ans;    cout << ans << endl;    return 0;}