Friend

Problem Description
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.

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

Output
For 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!

不一定对，你试试吧。

 #include<stdio.h>
#include<math.h>

int main(void) {
    long long number;

    while(scanf("%lld", &number) != EOF) {
        number++;
        int result = 0;
        for(int i = 0; i <= 30; i++) {
            if(pow(2, i) > number) {
                break;
            }
            double r = log(number / (double) pow(2, i)) / log(3.0);
            long long llr = (long long) r;
            if(r - llr < 1e-9 && r - llr > -1e-9) {
                result = 1;
            }
        }

        if(result) {
            printf("YES!\n");
        } else {
            printf("NO!\n");
        }
    }

    return 0;
}