380. Insert Delete GetRandom O(1)

Prompt

Implement the RandomizedSet class:

  • bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise.
  • bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise.
  • int getRandom() Returns a random element from the 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.
Input
["RandomizedSet", "insert", "remove", "insert", "getRandom", "remove", "insert", "getRandom"]
[[], [1], [2], [2], [], [1], [2], []]
Output
[null, true, false, true, 2, true, false, 2]

Explanation
RandomizedSet randomizedSet = new RandomizedSet();
randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomizedSet.remove(2); // Returns false as 2 does not exist in the set.
randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2].
randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly.
randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2].
randomizedSet.insert(2); // 2 was already in the set, so return false.
randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.
  • At most 2 * 105 calls will be made to insert, remove, and getRandom.
  • There will be at least one element in the data structure when getRandom is called.

Approach

For this problem, it would be instinctive to initially think to use a Hash Set as the problem statement repeatedly mentions in order to satisfy the constraint of implementing each of the class methods such that each function works in average O(1) time complexity. This is because the operations of search, insert, and delete for a Hash Set takes expected O(1) time. Thus, if we were asked to implement only the first 2 methods of insert & remove, we can sufficiently satisfy the constraint by implementing a Hash Set but unfortunately this is not the case.

Code

Python | Time: O(1) | Space: O(n)

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John Kim

John Kim

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