Merge two binary Max Heaps using C++.

Problem statement

Given two binary max heaps as arrays, merge the into single max heap.

Heap1[] = {20, 17, 15, 10}
Heap2[] = {19, 13, 7}
Result[] = {20, 19, 15, 13, 17, 7, 10}

Algorithm

1.Create an array to store result
2. Copy both given arrays one by one to result array
3. Build heap to construct full merged max heap

Example

#include <iostream>
#include <algorithm>
#define SIZE(arr) (sizeof(arr) / sizeof(arr[0]))
using namespace std;
void heapify(int *arr, int n, int idx){
   if (idx >= n) {
      return;
   }
   int l = 2 * idx + 1;
   int r = 2 * idx + 2;
   int max;
   if (l < n && arr[l] > arr[idx]) {
      max = l;
   } else {
      max = idx;
   }
   if (r < n && arr[r] > arr[max]) {
      max = r;
   }
   if (max != idx) {
      swap(arr[max], arr[idx]);
      heapify(arr, n, max);
   }
}
void createMaxHeap(int *arr, int n){
   for (int i = n / 2 - 1; i >= 0; --i) {
      heapify(arr, n, i);
   }
}
void mergeMaxHeaps(int *arr1, int n1, int *arr2, int n2, int *result){
   merge(arr1, arr1 + n1, arr2, arr2 + n2, result);
   createMaxHeap(result, n1 + n2);
}
void displayHeap(int *arr, int n){
   for (int i = 0; i < n; ++i) {
     cout << arr[i] << " ";
   }
   cout << endl;
}
int main(){
   int heap1[] = {20, 17, 15, 10};
   int heap2[] = {19, 13, 7};
   int result[SIZE(heap1) + SIZE(heap2)];
   cout << "First max heap: " << endl;
   displayHeap(heap1, SIZE(heap1));
   cout << "Second max heap: " << endl;
   displayHeap(heap2, SIZE(heap2));
   mergeMaxHeaps(heap1, SIZE(heap1), heap2, SIZE(heap2), result);
   cout << "Merged max heap: " << endl;
   displayHeap(result, SIZE(result));
   return 0;
}

Output

When you compile and execute the above program. It generates the following output −

First max heap:
20 17 15 10
Second max heap:
19 13 7
Merged max heap:
20 19 15 13 17 7 10