R help - Jul 2015 - Matrix Manipulation R

Hi guys,

Suppose I have an extremely large data frame with 2 columns and .5 mil
rows. For example, the last 6 rows may look like this:
.
..
...
89         100
93         120
95         125
101        NA
115        NA
123        NA
124        NA

I would like to manipulate this data frame to output a data frame that
looks like:,

100        89, 93, 95
120        101, 115
125        123, 124

What would be the absolute quickest way to do this, given that there are
many rows? Currently I have this:

# m is the large two column data frame
end <- na.omit(m[,'V2']);
out <- data.frame(End=end,
Start=unname(sapply(split(m[,'V1'],findInterval(m[,'V1'],end))[as.character(0:c(length(end)-1))],paste,collapse='.')))

However this is taking a little bit too long.

Thank you for your help!

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