Design a data structure that supports all following operations in average O(1) time.
Note: Duplicate elements are allowed.


insert(val): Inserts an item val to the collection.
remove(val): Removes an item val from the collection if present.
getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.



Example:

// Init an empty collection.
RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();

Difficulty: interview

Starter code:

class RandomizedCollection:
    def __init__(self):
        """
        Initialize your data structure here.
        """
    def insert(self, val: int) -> bool:
        """
        Inserts a value to the collection. Returns true if the collection did not already contain the specified element.
        """
    def remove(self, val: int) -> bool:
        """
        Removes a value from the collection. Returns true if the collection contained the specified element.
        """
    def getRandom(self) -> int:
        """
        Get a random element from the collection.
        """
# Your RandomizedCollection object will be instantiated and called as such:
# obj = RandomizedCollection()
# param_1 = obj.insert(val)
# param_2 = obj.remove(val)
# param_3 = obj.getRandom()

Solutions:
import random
 class RandomizedCollection:
 
     def __init__(self):
         """
         Initialize your data structure here.
         """
         self.val = []
         self.idx = {}
 
     def insert(self, val):
         """
         Inserts a value to the collection. Returns true if the collection did not already contain the specified element.
         :type val: int
         :rtype: bool
         """
         self.val.append(val)
         if val in self.idx:
             self.idx[val].append(len(self.val) - 1)
             return False
         else:
             self.idx[val] = [len(self.val) - 1]
 
     def remove(self, val):
         """
         Removes a value from the collection. Returns true if the collection contained the specified element.
         :type val: int
         :rtype: bool
         """
         if val not in self.idx:
             return False
         #print(self.val, self.idx)
         self.val[self.idx[val][-1]] = self.val[-1]
         if self.idx[self.val[-1]][-1] != self.idx[val][-1]:
             self.idx[self.val[-1]].pop()
             self.idx[self.val[-1]].append(self.idx[val][-1])
             self.idx[self.val[-1]].sort()
         self.val.pop()
         self.idx[val].pop()
         if len(self.idx[val]) == 0:
             del self.idx[val]
         return True
 
     def getRandom(self):
         """
         Get a random element from the collection.
         :rtype: int
         """
         return random.choice(self.val)
 
 
 # Your RandomizedCollection object will be instantiated and called as such:
 # obj = RandomizedCollection()
 # param_1 = obj.insert(val)
 # param_2 = obj.remove(val)
 # param_3 = obj.getRandom()

import random
 class RandomizedCollection(object):
     def __init__(self):
         self.l = []
         self.d = collections.defaultdict(set)
 
     def insert(self, val):
         b = val not in self.d
         self.d[val].add(len(self.l))
         self.l.append(val)
         return b
 
     def remove(self, val):
         if val not in self.d:
             return False
         i, newVal = self.d[val].pop(), self.l[-1]
         if len(self.d[val]) == 0:
             del self.d[val]
         self.l[i] = newVal
         if newVal in self.d:
             self.d[newVal].add(i)
             self.d[newVal].discard(len(self.l)-1)
         self.l.pop()
         return True
 
     def getRandom(self):
         return random.choice(self.l)