C# || How To Implement RandomizedSet – Insert Delete GetRandom O(1) Using C#

The following is a module with functions which demonstrates how to implement RandomizedSet – Insert Delete GetRandom O(1) using C#.


1. RandomizedSet – Problem Statement

Implement the RandomizedSet class:

  • RandomizedSet() Initializes the RandomizedSet object.
  • 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.

You must implement the functions of the class such that each function works in average O(1) time complexity.

Example 1:


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.


2. RandomizedSet – Solution

The following is a solution which demonstrates how to implement RandomizedSet – Insert Delete GetRandom O(1).

The main idea of this solution is to use a list to store the values added, and use a map to determine if an item has been added already.

The map is also used to store the list index of the added item. This makes it so we know which index to work with when we want to remove an item from the list.

To ensure proper removal, we ‘swap’ places of the value to remove with the last item in the list. This makes it so only the last item in the list is always the index to have items removed from.

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Once compiled, you should get this as your output for the example cases:


[null,true,false,true,1,true,false,2]

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