What is a Hash?

Key Values
animals rabbit, dog
fruits apple, watermelon, pineapple
letters A, B, B, F
continents Asia, Europe, Africa 
(Key, Value)
  - hashTable.put(key, value)   // returns true
  - hashTable.get(key)          // returns value, or NULL
  - hashTable.contains(value)   // returns true or false
  - hashTable.containsKey(key)  // returns true or false
  - hashTable.isEmpty()         // returns true or false
  - hashTable.size()            // returns int

Uses a hash function to map a key to a specific place in memory (array). That memory is then set to ‘value’.

1) Count number of unique elements in array

Given an integer array, return the number of unique integers in the array.

Input: [1,2,3,2,1,2,2,1]
Output: 3

Input: [1,2,3,4,5]
Output: 5

2) Two Sum

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

Input: nums = [2,7,11,15], target = 9
Output: [0,1]

Explanation: Since nums[0] + nums[1] == 9, we return [0, 1].

Input: nums = [3,2,4], target = 6
Output: [1,2]

Input: nums = [3,3], target = 6
Output: [0,1]

3) Check if sentence is pangram

Given a sentence (string), return true if the sentence is a pangram, false otherwise.

A pangram is a sentence that contains at least one occurence of every letter in the english language.

Input: "thequickbrownfoxjumpsoverthelazydog"
Output: true

Input: "The quick brown fox jumps over the lazy dog"
Output: true

Input: "PANDA"
Output: false

Input: "Sphinx of black quartz judge my vow"
Output: true

4) Count number of good pairs in array

Given an array of integers, return the number of good pairs.

A good pair is a pair (i, j) that meets the following criteria:

  • nums[i] == nums[j]
  • i < j
    • This prevents duplicate pairs. For example (i, j) is valid, but (j, i) isn’t.
Input: nums = [1,2,3,1,1,3]
Output: 4

Explanation: There are 4 good pairs (nums[0], nums[3]), (nums[0], nums[4]), (nums[3], nums[4]), (nums[2], nums[5]).

Input: nums = [1,1,1,1]
Output: 6

Explanation: Each pair in the array are good

Input: nums = [1,2,3]
Output: 0

5) Count numbers smaller than current number

Given the array nums, for each nums[i] find out how many numbers in the array are smaller than it. That is, for each nums[i] you have to count the number of valid j’s such that j != i and nums[j] < nums[i].

Return the answer in an array.

Input: nums = [8,1,2,2,3]
Output: [4,0,1,1,3]

Explanation:

  • nums[0]=8 has four smaller numbers than it: (1, 2, 2, 3)
  • nums[1]=1 has zero smaller numbers than it: (0)
  • nums[2]=2 has one smaller number than it: (1)
  • nums[3]=2 has one smaller number than it: (1)
  • nums[4]=3 has three smaller numbers than it: (1, 2, 2)
Input: nums = [6,5,4,8]
Output: [2,1,0,3]

Input: nums = [7,7,7,7]
Output: [0,0,0,0]