Amazon Software Developer I interview questions

Applied online through the Amazon.com college recruit link, after about a month a recruiter contacted me, and set up a phone interview after few days. When the guy called me he was very nice, and after the first few general questions ( I.E Why Amazon? ), he went into the technical questions, about OOP and basic Data Structures. He asked one thinking question: What is the best way to find out which number from 1 to 52 is missing from a 51 number array after a day i got a rejection from the recruiter

Applied on the site directly. Started off by asking simple questions relating to polymorphism, inheritance, protected&private access specifiers. Next, discussed about various data structures - array, linked list, hash tables - their complexity , space requirements etc. 1 question in coding - check if all elements in one strings exist in 2nd string. Design question - design a deck of cards. I don't know why I dint get through