def flip_transpose(arr):
    """
    Flip a 2D-list (i.e. transpose).
    """
    m = len(arr)
    n = len(arr[0])
    res = [[-1 for _ in range(n)] for _ in range(m)]

    for i in range(m):
        for j in range(n):
            res[i][j] = arr[j][i]
    return res