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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3. int n;
  4. long long int a[100005];
  5.  
  6. int main()
  7. {    
  8.     int t;
  9.     scanf("%d",&t);
  10.     while(t--)
  11.     {   scanf("%d",&n);
  12.         long long int sum=0;
  13.         for(int i=1;i<=n;i++)
  14.        {
  15. 		 scanf("%lld",&a[i]);
  16. 		 sum+=a[i];
  17. 	    }
  18.  
  19.  
  20.         if(n==2)
  21.         {   printf("%lld\n",min(a[1],a[2]));
  22. 		}
  23. 		else
  24.        {  
  25.  
  26.           set<long long int> s;
  27. 		 for(int i=1;i<=n;i++)
  28.         {    long long int A,D;
  29.              if(i==1)
  30.              {  A=a[i+1];
  31.                 D=a[i+2]-a[i+1];
  32.                 sum-=a[i];
  33.                 if((a[n]==(A+D*(n-2)))&&(sum==(((n-1)*(A+a[n]))/2)))
  34.                 {   s.insert(a[i]);
  35. 				}
  36. 				sum+=a[i];
  37.  
  38. 			 }
  39. 			 else if(i==n)
  40. 			 {    A=a[1];
  41. 			      D=a[i-1]-a[i-2];
  42. 			      sum-=a[i];
  43. 			      if((a[i-1]==(A+D*(n-2)))&&(sum==(((n-1)*(2*A+(n-2)*D))/2)))
  44. 			      {
  45. 				    s.insert(a[i]);
  46. 				  }
  47. 				  sum+=a[i];
  48. 			 }
  49. 			 else
  50. 			 {   A=a[1];
  51. 			     D=a[i+1]-a[i-1];
  52. 			     sum-=a[i];
  53. 			     if((a[n]==(A+D*(n-2)))&&(sum==(((n-1)*(2*A+(n-2)*D))/2)))
  54. 			     {  s.insert(a[i]);
  55. 				 }
  56. 				 sum+=a[i];
  57. 			 }
  58. 		}
  59. 		if(s.empty())
  60. 		{   printf("-1\n");
  61. 		}
  62. 		else
  63. 		{    set<long long int>:: iterator it;
  64. 		    it=s.begin();
  65. 		    printf("%lld\n",*it);
  66. 		}
  67. 	   }
  68. 	}
  69. return 0;
  70. }
Success #stdin #stdout 0s 4256KB
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https://ideone.com/c9upAo
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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