Missing number program in c++

[avishka45]

Is your feature request related to a problem? Please describe.
Want to add new program to c++ program foldr

Describe the solution you'd like
Missing number problem

Describe alternatives you've considered

Additional context
It is a famous array problem

[avishka45]

@fineanmol pls assign

[thrustyvector]

@fineanmol kindly describe the problem & please assign it to me

[SAMAKSH10]

@fineanmol kindly give more details

[vermasaurav]

can you please assign to me , i want to fix these issue!!