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



insert(val): Inserts an item val to the set if not already present.
remove(val): Removes an item val from the set if present.
getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.



Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

Source: leetcode | Difficulty: MEDIUM_HARD
Tags: Data structures

Mathematics

Probability

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

Solutions:
class RandomizedSet:

	def __init__(self):
		self.dic = {}
		self.l = []

	def insert(self, val):
		if val in self.dic:
			return False
		self.l.append(val)
		self.dic[val] = len(self.l) - 1
		return True

	def remove(self, val):
		if val not in self.dic:
			return False
		self.dic[self.l[-1]] = self.dic[val]
		(self.l[self.dic[val]], self.l[-1]) = (self.l[-1], self.l[self.dic[val]])
		self.l.pop()
		del self.dic[val]
		return True

	def getRandom(self):
		return random.choice(self.l)