This function creates an List of lists. that represents a two-dimensional grid with the given number of rows and cols. This grid should contain the integers from start to start + rows * cols - 1 in ascending order, but the elements of every odd-numbered row have to be listed in descending order, so that when read in ascending order, the numbers zigzag through the two-dimensional grid.

Input: Two ints rows and cols. One optional argument start.

Output: List of lists.