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.

Python Solution

Use hashmap and list together to support operations efficiently.

class RandomizedSet:

    def __init__(self):
        self.nums = []
        self.val_to_index = {}        

    def insert(self, val: int) -> bool:
        if val in self.val_to_index:
            return False

        self.val_to_index[val] = len(self.nums) - 1

        return True

    def remove(self, val: int) -> bool:
        if val not in self.val_to_index:
            return False

        deleted_index = self.val_to_index[val]

        if deleted_index < len(self.nums) - 1:
            last_num = self.nums[-1]
            self.nums[deleted_index] = last_num
            self.val_to_index[last_num] = deleted_index

        del self.val_to_index[val]

        return True

    def getRandom(self) -> int:

        return self.nums[random.randint(0, len(self.nums) - 1)]

# 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()
  • Time Complexity: O(1)
  • Space Complexity: O(N)

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