381. Insert Delete GetRandom O(1) - Duplicates allowed

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();

Solution: HashMap & HashSet

import random


class RandomizedCollection(object):
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.vals = list()
        self.idx = dict()

    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
        """
        idx = len(self.vals)
        self.vals.append(val)
        if val in self.idx:
            self.idx[val].add(idx)
            return False
        self.idx[val] = {idx}
        return True

    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 in self.idx:
            idx = self.idx[val].pop()
            last = self.vals[-1]
            self.vals[idx] = last
            self.vals.pop()
            self.idx[last].add(idx)
            self.idx[last].remove(len(self.vals))
            if not self.idx[val]:
                del self.idx[val]
            return True
        return False

    def getRandom(self):
        """
        Get a random element from the collection.
        :rtype: int
        """
        return random.choice(self.vals)

Related:

• 380. Insert Delete GetRandom O(1)