Insert Delete GetRandom O(1) in Python

Suppose we have a data structure that supports all following operations in average O(1) time.

• insert(val) − This will Insert an item val to the set if that is not already present.

• remove(val) − This will remove an item val from the set if it presents.

• getRandom − This will return a random element from current set of elements. Each element must have the same probability of being returned.

To solve this, we will follow these steps −

• For the initialization, define a parent map, and elements array

• For the insert() function, it will take val as input

  • if val is not present in parent map, or parent[val] = 0, then insert val into elements, and set parent[val] := 1, and return true

• return false

• For the remove() method, it will take val to remove

• if val is not present in parent map, or parent[val] = 0, then return false

• set parent[val] := 0

• index := index of val in elements array

• if index is not same as length of elements – 1

  • temp := last element of elements

  • set last element of elements := val

  • set elements[index] := temp

• delete last element of elements

• The getRandom() method will return any random value present in elements array

Example(Python)

Let us see the following implementation to get a better understanding −

 Live Demo

import random
class RandomizedSet(object):
   def __init__(self):
      self.present = {}
      self.elements = []
   def insert(self, val):
      if val not in self.present or self.present[val] == 0:
         self.elements.append(val)
         self.present[val] = 1
         return True
      return False
   def remove(self, val):
      if val not in self.present or self.present[val] == 0:
         return False
      self.present[val] = 0
      index = self.elements.index(val)
      if index!=len(self.elements)-1:
         temp = self.elements[-1]
         self.elements[-1] = val
         self.elements[index]=temp
      self.elements.pop()
      return True
   def getRandom(self):
      return random.choice(self.elements)
ob = RandomizedSet()
print(ob.insert(1))
print(ob.remove(2))
print(ob.insert(2))
print(ob.getRandom())
print(ob.remove(1))
print(ob.insert(2))
print(ob.getRandom())

Input

Initialize the class, then call the insert(), remove(), getRandom() functions. See the implementation.

Output

True
False
True
2
True
False
2