Oracle Software Engineer interview questions

Briefly explain primary key and foreign key

Given a number ‘n’ and a digit ‘k’. Each time, remove k digits from the given number and find the least possible number from the remaining digits. Continue to do this till the number of digits in ‘n’ becomes less than or equal to ‘k’. Finally return the smallest possible number. If the given number ‘n’ has number of digits less than ‘k’, then return the same number