Friend

HDU - 1719

Friend number are defined recursively as follows.
(1) numbers 1 and 2 are friend number;

(2) if a and b are friend numbers, so is ab+a+b;

(3) only the numbers defined in (1) and (2) are friend number.

Now your task is to judge whether an integer is a friend number.

InputThere are several lines in input, each line has a nunnegative integer a, 0<=a<=2^30.

OutputFor the number a on each line of the input, if a is a friend number, output “YES!”, otherwise output “NO!”.

Sample Input

3
13121
12131

Sample Output

YES!
YES!
NO! OJ-ID:
HDU-1719 author:
Caution_X date of submission:
20190930 tags:
数学推导 description modelling:
Friend number are defined recursively as follows.
(1) numbers 1 and 2 are friend number;
(2) if a and b are friend numbers, so is ab+a+b;
(3) only the numbers defined in (1) and (2) are friend number.
Now your task is to judge whether an integer is a friend number. major steps to solve it:
1.设n是Friend数,则n=ab+a+b=(a+1)*(b+1)-1   =>   n+1=(a+1)*(b+1)
2.a,b都是Friend数,所以a+1=(a'+1)*(b'+1),b+1=(a''+1)*(b''+1)直到a,b=1,2
3.那么:n+1= 2^x * 3^y warnings:
0需要特判 AC CODE:
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main()
{
ll n;
while(~scanf("%lld",&n)){
if(n==){
printf("NO!\n");
continue;
}
n++;
while(n%==) n/=;
while(n%==) n/=;
if(n==) printf("YES!\n");
else printf("NO!\n");
}
return ;
}



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