added all single number solution by c++

[harshsri28-zz]

i added single number I(136) , single number II(137), single number III(260) by c++.

if any formatting is to be done , kindly inform.

[qiyuangong]

Hi @harshsri28

Thank you for contribution!

Pls keep file name the same with existing files, i.e., 136_Single_Numberm 137_Single_Number_II and 260_Single_Number_III