Add LeetCode 1078. Occurrences After Bigram

[Tahanima]

Problem Link

[aniket-24]

Please assign this issue to me!

[Saikat2407]

would love to solve this issue!!

[JoyKrishan]

Hello apu,
I solved the problem #72 and would like to contribute :) .

[Tahanima]

@Saikat2407 and @JoyKrishan, I'm happy to accept both of your contributions. So, go ahead.