Question

I am coding this in C++

I have a BST filled with a class called Entry. Entry contains 2 members, a string and an int. The BST is sorted alphabetically by the string of each Entry. How do I take the 3 Entries with the highest int and put them into a vector from GREATEST to LEAST?

   // A helper class that stores a string and a frequency.
   class Entry
   {
   public:
       string s;
       int freq;
   };

   // A helper class that implements a BST node.
   class Node
   {
   public:
       Node()
       {
           left = right = nullptr;
       }

       Node(Entry e)
       {
           this->e = e;
           left = right = nullptr;
       }

       Entry e;
       Node* left;
       Node* right;
   };

Answer 1

All the explanation is in the code comments. hope this helps!

Code:

#include <iostream>

using namespace std;

// A helper class that stores a string and a frequency.
class Entry
{
public:
string s;
int freq;
};

// A helper class that implements a BST node.
class Node
{
public:
Node()
{
left = right = nullptr;
}

Node(Entry e)
{
this->e = e;
left = right = nullptr;
}

Entry e;
Node* left;
Node* right;
};

// function to insert string into BST
Node* insert(Node *head, string s) {
  
// empty node
if(head == nullptr)
{
Entry e = Entry();
e.s = s;
e.freq = 1;
head = new Node(e);
return head;
}
// equal node value
if(head->e.s == s) {
head->e.freq += 1;
}
else if(head->e.s < s) // go to right
head->right = insert(head->right, s);
else // go to left
head->left = insert(head->left, s);
return head;
}

// recursive function to traverse the BST to get strings with top 3 frequencies
void getMax(Node *head, Entry max[3])
{
// empty node
if(head == nullptr)
return;
  
// compare present node
for(int i=0; i<3; i++)
{
if(head->e.freq > max[i].freq) {
// shift all the entries from i to 1 place right
for(int j=2; j>i; j--)
{
max[j] = max[j-1];
}
// put present node at i
max[i] = head->e;
break;
}
}
  
// call for left and right trees
getMax(head->left, max);
getMax(head->right, max);
}

int main()
{
// sample run
// create a BST
Node *head = nullptr;
head = insert(head, "hello");
head = insert(head, "banana");
head = insert(head, "c++");
head = insert(head, "apple");
head = insert(head, "leaf");
head = insert(head, "apple");
head = insert(head, "apple");
head = insert(head, "book");
head = insert(head, "banana");
head = insert(head, "tree");
head = insert(head, "c++");
  
// array of first 3 max frequencies for strings
Entry max[3];
for(int i=0; i<3; i++) // initialization
{
max[i].s = "";
max[i].freq = 0;
}
  
// get top 3 frequency strings
getMax(head, max);
// output
cout << "String\t\tFrequency\n";
for(int i=0; i<3; i++)
{
cout << max[i].s << "\t\t" << max[i].freq << endl;
}

return 0;
}

Sample run:

Code screenshots: