LeetCode 380. Insert Delete GetRandom O(1)



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

  1. insert(val): Inserts an item val to the set if not already present.
  2. remove(val): Removes an item val from the set if present.
  3. getRandom: Returns a random element from current set of elements (it’s guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.


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

// Inserts 1 to the set. Returns true as 1 was inserted successfully.

// Returns false as 2 does not exist in the set.

// Inserts 2 to the set, returns true. Set now contains [1,2].

// getRandom should return either 1 or 2 randomly.

// Removes 1 from the set, returns true. Set now contains [2].

// 2 was already in the set, so return false.

// Since 2 is the only number in the set, getRandom always return 2.


use an array and a hashmap

Python Solution

class RandomizedSet:

    def __init__(self):
        Initialize your data structure here.
        self.values_indexes = {}
        self.values = []

    def insert(self, val: int) -> bool:
        Inserts a value to the set. Returns true if the set did not already contain the specified element.
        if val not in self.values_indexes:
            self.values_indexes[val] = len(self.values)
            return True
            return False

    def remove(self, val: int) -> bool:
        Removes a value from the set. Returns true if the set contained the specified element.
        if val not in self.values_indexes:
            return False
            index = self.values_indexes[val]
            last = len(self.values) - 1
            self.values_indexes[self.values[last]] = index
            self.values[index], self.values[last] = self.values[last], self.values[index]
            del self.values_indexes[val]
            return True

    def getRandom(self) -> int:
        Get a random element from the set.
        random_index = random.randint(0, len(self.values) - 1)        

        return self.values[random_index]

# 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()

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