codeforces #371div2 B

 1     #include <bits/stdc++.h>  
 2     #define INF 0x3f3f3f3f  
 3     #define N 100000+5  
 4     typedef long long ll;  
 5     using namespace std;  
 6       
 7     int a[N];  
 8     int main()  
 9     {  
10         ios_base::sync_with_stdio(0);  
11         cin.tie(0);  
12       
13         int n;  
14         while(cin>>n){  
15             int ok = 1;  
16             for(int i = 1;i <= n;++ i)  
17                 cin>>a[i];  
18       
19             sort(a+1,a+n+1);  
20             int len = unique(a+1,a+n+1)-a;  
21             len--;  
22             if(len > 3)  
23                 ok = 0;  
24             else{  
25                 if(len == 3){  
26                     if(abs(a[1]-a[2]) != abs(a[2]-a[3]))  
27                         ok = 0;  
28                 }  
29             }  
30             if(ok)  cout<<"YES"<<endl;  
31             else  
32                 cout<<"NO"<<endl;  
33         }  
34         return 0;  
35     }
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