C++: Fastest method to check if all array elements are equal

What is the fastest method to check if all elements of an array(preferable integer array) are equal. Till now I have been using the following code:

bool check(int array[], int n)
{   
    bool flag = 0;

    for(int i = 0; i < n - 1; i++)      
    {         
        if(array[i] != array[i + 1])
            flag = 1;
    }

    return flag;
}

int check(const int a[], int n)
{   
    while(--n>0 && a[n]==a[0]);
    return n!=0;
}

Here is a solid solution which is valid C++11. The advantages is that you do not need to manually play with the indexes or iterators. It is a best practice to

prefer algorithm calls to handwritten loops [Herb Sutter - C++ Coding Standards]

I think this will equally efficient as Paul R's solution.

bool check(const int a[], int n)
{
      return !std::all_of(a, a+n, [a](int x){ return x==a[0]; });
}

Once you have found a mismatching element you can break out of the loop:

bool check(const int array[], int n)
{   
    for (int i = 0; i < n - 1; i++)      
    {         
        if (array[i] != array[i + 1])
            return true;
    }
    return false;
}

If this is performance-critical then it can be further optimised slightly as:

bool check(const int array[], int n)
{   
    const int a0 = array[0];

    for (int i = 1; i < n; i++)      
    {         
        if (array[i] != a0)
            return true;
    }
    return false;
}

Recast the array to a larger data type. Eg, operate on 64bit ints, or use SSE or AVX intrinsics for 128 or 256 bit operation. For example, the SSE2 intrinsic is _mm_cmpeq_epi32, whose result you'll use with _mm_or_si128. Check the result with repeated application of _mm_srli_si128 and _mm_cvtsi128_si32. Check the result every few hundred iterations for early exit.

Make sure to operate on aligned memory, check the unaligned start and end as ints, and check the first packed element with itself.