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

October 20, 2021 admin No comments

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.





		
		
			2. RandomizedSet - Solution
			
C#
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Oct 20, 2021
//    Taken From: http://programmingnotes.org/
//    File:  Solution.cs
//    Description: Demonstrates how to implement RandomizedSet
// ============================================================================
public class RandomizedSet {
    // For randomizing the list
    private Random r;

    // To determine if an item has been added, and store its list index
    private Dictionary<int, int> map;

    // To store the items added
    private List<int> values;

    public RandomizedSet() {
        r = new Random();
        map = new Dictionary<int, int>();
        values = new List<int>();
    }

    // Inserts a value to the set. Returns true if the set did not already contain the specified element
    public bool Insert(int val) {
        // Determine if item exists in map
        if (map.ContainsKey(val)) {
            return false;
        }

        // Add value to the list
        values.Add(val);

        // Save the value with its list index to the map
        map[val] = values.Count - 1;

        return true;
    }

    // Removes a value from the set. Returns true if the set contained the specified element
    public bool Remove(int val) {
        // Determine if item exists in map
        if (!map.ContainsKey(val)) {
            return false;
        }

        // Get the current index for the item to remove
        var currentIndex = map[val];

        // Get the last index for the item at the end
        var lastIndex = values.Count - 1;

        // 'Swap' places of the last item in the list with the item to be removed
        values[currentIndex] = values[lastIndex];

        // Update the map for the item that we 'swapped' with its new list index
        map[values[currentIndex]] = currentIndex;

        // Remove the swapped item from the list (this is the item to be removed)
        values.RemoveAt(lastIndex);

        // Remove the item to be removed from the map
        map.Remove(val);

        return true;
    }

    // Get a random element from the set
    public int GetRandom() {
        var randomIndex = r.Next(0, values.Count);
        return values[randomIndex];
    }
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Oct 20, 2021
//    Taken From: http://programmingnotes.org/
//    File:  Solution.cs
//    Description: Demonstrates how to implement RandomizedSet
// ============================================================================
public class RandomizedSet {
    // For randomizing the list
    private Random r;
 
    // To determine if an item has been added, and store its list index
    private Dictionary<int, int> map;
 
    // To store the items added
    private List<int> values;
 
    public RandomizedSet() {
        r = new Random();
        map = new Dictionary<int, int>();
        values = new List<int>();
    }
 
    // Inserts a value to the set. Returns true if the set did not already contain the specified element
    public bool Insert(int val) {
        // Determine if item exists in map
        if (map.ContainsKey(val)) {
            return false;
        }
 
        // Add value to the list
        values.Add(val);
 
        // Save the value with its list index to the map
        map[val] = values.Count - 1;
 
        return true;
    }
 
    // Removes a value from the set. Returns true if the set contained the specified element
    public bool Remove(int val) {
        // Determine if item exists in map
        if (!map.ContainsKey(val)) {
            return false;
        }
 
        // Get the current index for the item to remove
        var currentIndex = map[val];
 
        // Get the last index for the item at the end
        var lastIndex = values.Count - 1;
 
        // 'Swap' places of the last item in the list with the item to be removed
        values[currentIndex] = values[lastIndex];
 
        // Update the map for the item that we 'swapped' with its new list index
        map[values[currentIndex]] = currentIndex;
 
        // Remove the swapped item from the list (this is the item to be removed)
        values.RemoveAt(lastIndex);
 
        // Remove the item to be removed from the map
        map.Remove(val);
 
        return true;
    }
 
    // Get a random element from the set
    public int GetRandom() {
        var randomIndex = r.Next(0, values.Count);
        return values[randomIndex];
    }
}// http://programmingnotes.org/

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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Posted in: C#, Console Tags: c-sharp, leetcode

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

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