Van Emde Boas Tree | Set 2 | Insertion, Find, Minimum and Maximum Queries

It is highly recommended to see previous articles on Van Emde Boas Tree first.

Procedure for Insert :

  1. If no keys are present in the tree then simply assign minimum and maximum of the tree to the key.
  2. Otherwise we will go deeper in the tree and do the following:
    • If the key we want to insert is less than the current minimum of the tree, then we swap both values because the new key will be the real minimum of the tree and the key which was already at the place of the minimum will be used for the further process.

      This concept can be thought of as lazy propagation in Van Emde Boas Tree. Because this old minimum value really is a minimum of one of the clusters of recursive Van Emde Boas Structure. So actually we don’t go deeper into the structure until the need arises.

    • If we are not at the base case means universe size of the tree is greater than 2 then :

      • If the tree’s cluster[High(key)] is empty, then we recursively call insert over the summary and as we are doing lazy propagation, we just assign minimum and maximum value to the key and stop the recursion.
      • Otherwise, we call insert over the cluster in which the key is present.
  3. Similarly, We check for maximum and set the key as maximum if it is greater than the current maximum.

Below Image represents empty VEB(4) Tree:

VEB Tree

Now we insert 1, then it will just set the minimum and maximum of the tree to 1. You can see the Lazy propagation of 1:

VEB Tree

Now if we insert 0, then 1 will propagate to the 1st cluster and zero will be the new minimum:
VEB

Procedure for isMember Query :

  • At any point of our search, If the key is minimum or maximum of the tree, which means the key is present, then return true.
  • If we reach the base case, but above condition is false then the key must not be present in the tree so return true.
  • Otherwise recursively call the function over the cluster of the key i.e.(High(key)) and its position in the cluster i.e.(Low(key)).
  • Here we are allowing universe_size to be any power of 2, so that if the situation arises in which universe_size is less than the key value then return false.

Minimum & Maximum : Van Emde Boas Tree stores minimum and maximum as its attributes, so we can return its value if it is present and null otherwise.

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#include <bits/stdc++.h>
using namespace std;
  
class Van_Emde_Boas {
  
public:
    int universe_size;
    int minimum;
    int maximum;
    Van_Emde_Boas* summary;
    vector<Van_Emde_Boas*> clusters;
  
    // Function to return cluster numbers
    // in which key is present
    int high(int x)
    {
        int div = ceil(sqrt(universe_size));
        return x / div;
    }
  
    // Function to return position of x in cluster
    int low(int x)
    {
        int mod = ceil(sqrt(universe_size));
        return x % mod;
    }
  
    // Function to return the index from
    // cluster number and position
    int generate_index(int x, int y)
    {
        int ru = ceil(sqrt(universe_size));
        return x * ru + y;
    }
  
    // Constructor
    Van_Emde_Boas(int size)
    {
        universe_size = size;
        minimum = -1;
        maximum = -1;
  
        // Base case
        if (size <= 2) {
            summary = nullptr;
            clusters = vector<Van_Emde_Boas*>(0, nullptr);
        }
        else {
            int no_clusters = ceil(sqrt(size));
  
            // Assigning VEB(sqrt(u)) to summary
            summary = new Van_Emde_Boas(no_clusters);
  
            // Creating array of VEB Tree pointers of size sqrt(u)
            clusters = vector<Van_Emde_Boas*>(no_clusters, nullptr);
  
            // Assigning VEB(sqrt(u)) to all its clusters
            for (int i = 0; i < no_clusters; i++) {
                clusters[i] = new Van_Emde_Boas(ceil(sqrt(size)));
            }
        }
    }
};
  
// Function to return the minimum value
// from the tree if it exists
int VEB_minimum(Van_Emde_Boas* helper)
{
    return (helper->minimum == -1 ? -1 : helper->minimum);
}
  
// Function to return the maximum value
// from the tree if it exists
int VEB_maximum(Van_Emde_Boas* helper)
{
    return (helper->maximum == -1 ? -1 : helper->maximum);
}
  
// Function to insert a key in the tree
void insert(Van_Emde_Boas* helper, int key)
{
    // If no key is present in the tree
    // then set both minimum and maximum
    // to the key (Read the previous article
    // for more understanding about it)
    if (helper->minimum == -1) {
        helper->minimum = key;
        helper->maximum = key;
    }
    else {
        if (key < helper->minimum) {
  
            // If the key is less than current minimum
            // then swap it with the current minimum
            // because this minimum is actually
            // minimum of one of the internal cluster
            // so as we go deeper into the Van Emde Boas
            // we need to take that minimum to its real position
            // This concept is similar to "Lazy Propagation"
            swap(helper->minimum, key);
        }
  
        // Not base case then...
        if (helper->universe_size > 2) {
  
            // If no key is present in the cluster then insert key into
            // both cluster and summary
            if (VEB_minimum(helper->clusters[helper->high(key)]) == -1) {
                insert(helper->summary, helper->high(key));
  
                // Sets the minimum and maximum of cluster to the key
                // as no other keys are present we will stop at this level
                // we are not going deeper into the structure like
                // Lazy Propagation
                helper->clusters[helper->high(key)]->minimum = helper->low(key);
                helper->clusters[helper->high(key)]->maximum = helper->low(key);
            }
            else {
                // If there are other elements in the tree then recursively
                // go deeper into the structure to set attributes accordingly
                insert(helper->clusters[helper->high(key)], helper->low(key));
            }
        }
  
        // Sets the key as maximum it is greater than current maximum
        if (key > helper->maximum) {
            helper->maximum = key;
        }
    }
}
  
// Function that returns true if the
// key is present in the tree
bool isMember(Van_Emde_Boas* helper, int key)
{
  
    // If universe_size is less than the key
    // then we can not search the key so returns
    // false
    if (helper->universe_size < key) {
        return false;
    }
  
    // If at any point of our traversal
    // of the tree if the key is the minimum
    // or the maximum of the subtree, then
    // the key is present so returns true
    if (helper->minimum == key || helper->maximum == key) {
        return true;
    }
    else {
  
        // If after attending above condition,
        // if the size of the tree is 2 then
        // the present key must be
        // maximum or minimum of the tree if it
        // is not then it returns false becuase key
        // can not be present in the sub tree
        if (helper->universe_size == 2) {
            return false;
        }
        else {
  
            // Recursive call over the cluster
            // in which the key can be present
            // and also pass the new position of the key
            // i.e., low(key)
            return isMember(helper->clusters[helper->high(key)],
                            helper->low(key));
        }
    }
}
  
// Driver code
int main()
{
    Van_Emde_Boas* veb = new Van_Emde_Boas(8);
  
    // Inserting Keys
    insert(veb, 2);
    insert(veb, 3);
    insert(veb, 6);
  
    cout << boolalpha;
  
    // Checking isMember query
    cout << isMember(veb, 3) << endl;
  
    cout << isMember(veb, 4) << endl;
  
    // Maximum of VEB
    cout << VEB_maximum(veb) << endl;
  
    // Minimum of VEB
    cout << VEB_minimum(veb) << endl;
}

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Output:

true
false
6
2


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I am doing BTech at Dhirubhai Ambani Institute of Information and Communication Technology

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Improved By : Aakash_Panchal