Exercise: Merge files with timestamps



examples/dictionary/merge/a.csv
1601009973,1
1601009975,3
1601009976,4
1601009978,6
1601009981,9
1601009982,10
1601009983,11
1601009984,12
1601009987,15
1601009989,17
1601009990,18
1601009991,19
1601009992,20


examples/dictionary/merge/b.csv
1601009974,2
1601009977,5
1601009980,8
1601009988,16


examples/dictionary/merge/c.csv
1601009979,7
1601009985,13
1601009986,14


examples/dictionary/merge/d.csv