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system
write some pseudo-code to implement merge sort.
user
merge sort(a, n) 
    if n == 1 
        return a 
    else 
        split a into two sub-arrays: a1 and a2 
        a = mergesort(a1, n/2) 
        b = mergesort(a2, n/2) 
        return merge(a, b) 
 
merge(a1, a2) 
    create an empty array: a 
    i = j = 0 
    while i < a1.length && j < a2.length 
        if a1[i] < a2[j] 
            a.append(a1[i]) 
            i++ 
        else 
            a.append(a2[j]) 
            j++ 
    add remaining elements of a1 and a2 to a 
    return a
assistant