D. Taxes
题目链接
http://codeforces.com/contest/735/problem/D
题面
Mr. Funt now lives in a country with a very specific tax laws. The total income of mr. Funt during this year is equal to n (n ≥ 2) burles and the amount of tax he has to pay is calculated as the maximum divisor of n (not equal to n, of course). For example, if n = 6 then Funt has to pay 3 burles, while for n = 25 he needs to pay 5 and if n = 2 he pays only 1 burle.
As mr. Funt is a very opportunistic person he wants to cheat a bit. In particular, he wants to split the initial n in several parts n1 + n2 + ... + nk = n (here k is arbitrary, even k = 1 is allowed) and pay the taxes for each part separately. He can't make some part equal to 1 because it will reveal him. So, the condition ni ≥ 2 should hold for all i from 1 to k.
Ostap Bender wonders, how many money Funt has to pay (i.e. minimal) if he chooses and optimal way to split n in parts.
输入
The first line of the input contains a single integer n (2 ≤ n ≤ 2·109) — the total year income of mr. Funt.
输出
Print one integer — minimum possible number of burles that mr. Funt has to pay as a tax.
样例输入
4
样例输出
2
题面
现在给你一个数n，他的代价就是他除开本身的最大因子。
现在你可以把n拆成几个数相加，这样代价就是那几个数的代价的和。
问你最小的代价是啥。
题解
拆成素数的话，答案就是1
根据哥德巴赫猜想，大于2的偶数都可以变成两个素数的和。
大于5d奇数，都可以拆成三个素数的和。
代码
#include<bits/stdc++.h>
using namespace std;
long long n;
bool check(int x){
for(int i=2;i*i<=x;i++){
if(x%i==0)return false;
}
return true;
}
int main()
{
scanf("%lld",&n);
if(n>2&&n%2==0){
printf("2\n");
}else if(n==2){
printf("1\n");
}else{
if(check(n))cout<<"1"<<endl;
else if(check(n-2))cout<<"2"<<endl;
else cout<<"3"<<endl;
}
}