To do it quickly you need to Sort the data. To be able to Sort, the Data must be in a "Table"
A Table is a group of Data which has continuous Header cells across the top and continuous Row cells along its left side. Any interruption in the Top row and Left column will be seen as the end of the Table.
IMPORTANT: the row immediately above the Table must be empty. Insert a Row just above the Table. Hide the row if you don't want to see it. (same rule for the column just to the left of the leftmost column, it must be empty)
Then Sort the Table. (you must 'select' a cell within the Table or Sort won't know what to sort)
BEFORE YOU SORT, be sure the leftmost column has values that will allow you to Unsort when you're done! Call the first Column 'Unsort' and fill it with numbers 1 to whatever.
And always SAVE before you Sort, in case it goes awry.
Sort the Table.
When all the empty cells are together, enter whatever you want in the topmost empty cell of a column.
Select the value you have just entered, hold Shift, press End then down arrow, (then Up arrow once to Unselect the next piece of data)
Ctrl + D is the fasted way to Fill Down the value in the topmost cell of the selection. Repeat for each column.
Unsort when done using the numerical series in the Leftmost column.