Borrowed the following article from http://get-that-job-at-google.blogspot.in/
Coding Knowledge C/C++ and Java are the preferred programming languages for Google Interviewers. You must know at least one of them really well. You will be expected to write code in the phone screen interviews and in the onsite interviews as well.
Recommended books for CS interviews:
Introduction To Algorithms - By Cormen
Programming Interviews Exposed
Cracking the Coding Interviews
Algorithms for Interviews
Recommended websites for coding practice: InterviewStreet, Topcoder
Big-O This should be the starting point in preparing for an algorithmic interview. You must not struggle with basic complexity analysis, as it will guarantee not being hired. You should be familiar and understand the O, Θ and Ω notations. I recommend reading section on complexity analysis from Data Structures and Algorithms book.
Sorting You should be able to write algorithms O(n*lgn) like QuickSort and MergeSort with ease. Compare and understand the best, worst and average case complexities. I found this table on wiki to be very handy; it lists important properties of all sorting algorithms. Don’t neglect the basic O(n^2) algorithms like Bubble sort or Insertion sort, since other algorithms improve over these. Interviews are more about improving a basic idea, sorting algorithms will help with this process.
Hash Tables When in doubt, think of hash tables. They are useful in most of the problems and frequently help us improve the time complexity of some problems by caching results.
Trees Go through basic tree construction, traversal and manipulation algorithms. You should be able to implement algorithms based on binary search trees. You should be familiar with balanced trees although you are not expected to write code for them in the interview: AVL trees, Red-Black trees, Trie, n-ary trees etc. Thorough knowledge about inorder, postorder and preorder traversals is necessary, because we can solve many tree problems by doing simple modifications to one of these traversals.
Graphs are a very important concept in Computer Science. Practice the three basic representation of graphs (objects and pointers, matrix, and adjacency list) and familiarize yourself with their pros & cons.
There is not much time during the interview so you should not expect something very complex. However, basic graph traversal algorithms (DFS and BFS) are a must, you should implement them in all basic representations.
You should be able to implement the Dijkstra or Floyd-Warshall algorithms as well as minimum spanning tree algorithms (Kruskal and Prim). Learn about topological sorting, since it is surprisingly very useful in many ordering problems.
Dynamic Programming This is probably the most important subject as the implementations are small. You should be able to implement 2-3 dynamic algorithms during a 35-40 minutes time. As you’ll check the resources on this blog or on the web, you’ll find that you should expect at least one dynamic programming question per interview.
Operating Systems Learn about processes, threads and concurrency issues. Know about mutexes, semaphores, monitors and how they work. Understand what deadlock and livelock are and how to avoid them. Learn about context switching, scheduling etc.
Mathematics You should familiarize yourself with counting, combinatorics and probability.
Google's publications Read up Google's path-breaking publications listed below if you have time.
Google File System