Finding index to values at latest dates in tall timetable

I have a tall timetable containing some repeated values (a, b, or c) in one column. I want to find the rows containing the latest time associated with a, with b, and with c and get rid of the non-latest rows in the tall timetable. Here's what I tried. Is there a better way?

%% Make some example data
nRow = 100;
letterList = {'a', 'b', 'c'};
stringList = randsample(letterList, nRow, true)'; % Column containing a, b or c
timeList = datetime(2017, 03, 1) + rand(nRow, 1)*1000; 
% Make a regular table
tableReg = table(timeList, ... % Date
    stringList, ... % List with values of either 'a', 'b', or 'c'
    rand(nRow, 1), ... % Value
    'VariableNames', {'Date', 'Letter', 'Value'});
% Make a tall table
tt = tall(tableReg);
nRow = gather(height(tt)); % Confirm there are now only 3 rows in tt after trimming
disp([num2str(nRow) ' rows originally'])

%% Get index to latest value of each letter
% Sort by time in descending order, so most recent is first
tt = sortrows(tt, 'Date', 'desc'); 
[~, iLatestLetter, ~] = unique(tt.Letter); % Default is first value
iLatestLetter = gather(iLatestLetter); 
tt = tt(iLatestLetter, :); % Trim table to only select latest values of each letter
nRow = gather(height(tt)); % Confirm there are now only 3 rows in tt after trimming
disp([num2str(nRow) ' rows after trim'])