A hash table is a data structure that maps keys to values for highly effient lookup.
Given an array of integers, find if the array contains any duplicates.
Given a string, find the length of the longest substring without repeating characters.
Find if a duplicate exists among the recent k numbers.
Given a digit string, return all possible letter combinations that the number could represent.
Write an algorithm to determine if a number is "happy".
Given an array of integers, every element appears twice except for one. Find that single one.
Given an array of integers, every element appears three times except for one. Find that single one.
Given an array of numbers nums, in which exactly two elements appear only once and all the other elements appear exactly twice. Find the two elements that appear only once.
Write a function to find all the 10-letter-long sequences (substrings) that occur more than once in a DNA molecule.
Design a data structure that supports add(word) and search(word).
Given two strings s and t, determine if they are isomorphic.
Given two strings s and t, write a function to determine if t is an anagram of s.
Given an array of size n, find the majority element. The majority element is the element that appears more than ⌊ n/2 ⌋ times.
Given a sudoku, determine if it is valid.
Given a pattern and a string str, find if str follows the same pattern.
Bulls and Cows is an interesting problem that could be solved by hash table in an interesting way.