Given two sorted arrays, A and B, and A has a large enough buffer at the end to hold B. Write a method to merge B into A in sorted order.

Solution# 1: We will start with three pointers: a_end, a_tail and b_tail where;
a_end - last memory location of A
a_tail - last non-zero element of A
b_tail - last non-zero element of B

We start the merge from the end by comparing elements at a_tail and b_tail and putting the greater one at a_end. Here is the code,

void mergeArrays(int* A, int* B, int a_size, int b_size)
{
    int a_end = a_size + b_size - 1;
    int a_tail = a_size - 1;
    int b_tail = b_size - 1;
    
    while((a_tail >= 0) && (b_tail >= 0))
    {
        if(B[b_tail] > A[a_tail])
            A[a_end--] = B[b_tail--];
        else
            A[a_end--] = A[a_tail--];
    }

    while(b_tail >=0)
        A[a_end--] = B[b_tail--];
}

void main()
{
    int A[200] = {0,};
    int B[50] = {0,};
    int i;

    for(i=80; i<230; i++)
        A[i-80] = i*2;

    for(i=0; i<50; i++)
        B[i] = i;

    mergeArrays(A, B, 150, 50);

    for(i=0; i<200; i++)
        printf("A[%d] =  %d\n", i, A[i]);

}