def mergeArrays(a1,a2,m,n):

    i, j, k = m-1, n-1, m+n-1

    while i >=0 and j >=0:
        if a1[i] >= a2[j]:
            i-=1

        else:
            a1.insert(i+1,a2[j])
            j-=1

    return a1

a = [1, 5, 9, 10, 15, 20]
b = [2, 3, 8, 13]
m,n = len(a), len(b)