A. Infinite Sequence
题目连接：
http://www.codeforces.com/contest/675/problem/A
Description
Vasya likes everything infinite. Now he is studying the properties of a sequence s, such that its first element is equal to a (s1 = a), and the difference between any two neighbouring elements is equal to c (si - si - 1 = c). In particular, Vasya wonders if his favourite integer b appears in this sequence, that is, there exists a positive integer i, such that si = b. Of course, you are the person he asks for a help.
Input
The first line of the input contain three integers a, b and c ( - 109 ≤ a, b, c ≤ 109) — the first element of the sequence, Vasya's favorite number and the difference between any two neighbouring elements of the sequence, respectively.
Output
If b appears in the sequence s print "YES" (without quotes), otherwise print "NO" (without quotes).
Sample Input
1 7 3
Sample Output
YES
Hint
题意
A0=a,Ai=Ai-1+b,问你c可不可能出现在这个无限长的数列里面
题解：
水题，用数学方法去做就好了
代码
#include<bits/stdc++.h>
using namespace std;
int main()
{
long long a,b,c;
cin>>a>>b>>c;
b-=a;
if(c==0&&b!=0)return puts("NO"),0;
if(c==0&&b==0)return puts("YES"),0;
if(b%c!=0)return puts("NO"),0;
else if(b/c<0)return puts("NO"),0;
return puts("YES"),0;
}