Python/divide_and_conquer/mergesort.py

def merge(a, b, m, e):
    l = a[b : m + 1]
    r = a[m + 1 : e + 1]
    k = b
    i = 0
    j = 0
    while i < len(l) and j < len(r):
        # change sign for Descending order
        if l[i] < r[j]:
            a[k] = l[i]
            i += 1
        else:
            a[k] = r[j]
            j += 1
        k += 1
    while i < len(l):
        a[k] = l[i]
        i += 1
        k += 1
    while j < len(r):
        a[k] = r[j]
        j += 1
        k += 1
    return a


def mergesort(a, b, e):
    """
    >>> mergesort([3,2,1],0,2)
    [1, 2, 3]
    >>> mergesort([3,2,1,0,1,2,3,5,4],0,8)
    [0, 1, 1, 2, 2, 3, 3, 4, 5]
    """
    if b < e:
        m = (b + e) // 2
        # print("ms1",a,b,m)
        mergesort(a, b, m)
        # print("ms2",a,m+1,e)
        mergesort(a, m + 1, e)
        # print("m",a,b,m,e)
        merge(a, b, m, e)
        return a


if __name__ == "__main__":
    import doctest

    doctest.testmod()