I am stumbling to produce a result on this one.

I have a collection of XLS Cells (15 x 10K). I'd like to determine the total UNIQUE values appearing within the array, and then the frequency with which each appears? I do not know the possible unique values in advance.

Simple Example (3 x 4):

A | B | C
B | B | A
C | C | C
D | A | B

Result:

A: 3
B: 4
C: 4
D: 1

Any suggestions to approach this problem?