Examples of Content related issues. PERFECT HASHING 56 10.5 Perfect Hashing We say a hash function is perfect for S if all lookups involve O(1) work. 200+ algorithmic coding problems. Software related issues. Idea A two level scheme with universal hashing at each level such that there are no collisions at level 2. With our complete interview preparation course, you’ll get access to precise content required for placement preparation, nothing less, nothing more. The two heuristic methods are hashing by division and hashing by multiplication which are as follows: The mod method: In this method for creating hash functions, we map a key into one of the slots of table by taking the remainder of key divided by table_size. Perfect Hashing Cichelli's Algorithm. Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube. See your article appearing on the GeeksforGeeks main page and help other Geeks. Premium Lecture videos by Mr. Sandeep Jain (CEO & Founder, GeeksforGeeks) and other industry experts key % table_size Your 4 years of learning compressed into an extensive course of 6 months! For queries regarding questions and quizzes, use the comment area below respective pages. Weekly assessment tests. Dedicated TAs for Doubt Assistance. 11:29. Skip navigation ... GeeksforGeeks - Duration: 11:29. If yes, then we have the perfect tool for you. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. Track-based learning. This way the hash function covers all your hash space uniformly. Given an array A[] and a number x, check for pair in A[] with sum as x; Hashing | Set 1 (Introduction) Hashing | Set 3 (Open Addressing) Count pairs with given sum Edit: The biggest disadvantage of this hash function is that it preserves divisibility, so if your integers are all divisible by 2 or by 4 (which is not uncommon), their hashes will be too. So in the particular case of the data set of the original question, your solution is satisfactory, but in the more general case of "finding a perfect hash function for data sets" (in particular, larger than some threshold), your answer isn't suitable. That is, the hash function is ; h(key) = key mod table_size i.e. This is where Perfect Hashing comes in, but there is a price to pay; the table must be static. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. GeeksforGeeks 122,786 views. 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