Shell Programming and Scripting

awk '{k=$2;a[k]=a[k]"\t"$1}END{for(k in a)print k,a[k]}' OFS="\t" filename

#!/usr/bin/env python
d={}
for line in open("file"):
    line=line.strip().split()
    d.setdefault(line[-1],[])
    d[line[-1]].append(line[0])
for i,j in d.iteritems():
    print i, '\t'.join(j)

YYYYYYYYY       a6      a62
ZZZZZ   a30     a13     a43     a23
XXXXXXX         a1      a2      a71

 
awk '{k=$2;a[k]=a[k]"\t"$1}END{for(k in a)print k,a[k]}' filename

$
$ perl -ne 'split; $x{$_[1]}=$x{$_[1]}."\t".$_[0]; END{foreach $k(keys %x){print $k,"\t",$x{$k},"\n"}}' input.txt
XXXXXXX         a1      a2      a71
ZZZZZ           a30     a13     a43     a23
YYYYYYYYY               a6      a62
$
$