SRM 387 DIV2 [250]

#include<iostream>
#include<vector>
using namespace std;

class GuessingNextElement
...{
public:
    int guess(vector<int> A)
    ...{
        if (A[1]-A[0]==A[2]-A[1])...{
            return A[A.size()-1]+A[1]-A[0];
        }
        else
            return A[A.size()-1]*(A[1]/A[0]);
    }
};

Problem Statement
	An integer arithmetic progression is a sequence defined by two positive integers, p and q, where p is the first element in the sequence, and all other elements are obtained by adding q to the previous element. For example, if p = 1 and q = 2, the sequence would be: 1, 3, 5, 7, ... An integer geometric progression is a sequence defined by two positive integers, p and q, where p is the first element in the sequence, and all other elements are obtained by multiplying the previous element by q. For example, if p = 3 and q = 2, the sequence would be: 3, 6, 12, ... You are given a vector <int> A, which contains either an integer arithmetic or geometric progression. Determine which one it is and return the next element in the sequence. It is guaranteed that A will uniquely represent either an arithmetic or geometric progression and that result will fit in a 32-bit signed integer.
Definition
	Class: GuessingNextElement Method: guess Parameters: vector <int> Returns: int Method signature: int guess(vector <int> A) (be sure your method is public)
Constraints
-	A will contain between 3 and 50 elements, inclusive.
-	Each element of A will be between 1 and 10^6, inclusive.
-	A will be sorted in ascending order.
-	A will uniquely represent either an arithmetic or geometric progression.
Examples
0)
	{364,843,1322,1801} Returns: 2280 This sequence represents an arithmetic progression where p = 364 and q = 479. The next element is 1801 + 479 = 2280.
1)
	{394,1172,1950,2728,3506,4284,5062,5840} Returns: 6618
2)
	{13,117,1053,9477,85293} Returns: 767637 This sequence represents a geometric progression where p = 13 and q = 9. The next element is 85293 * 9 = 76737.
3)
	{22,220,2200,22000} Returns: 220000
4)
	{250000, 500000, 1000000} Returns: 2000000

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