# Merge two binary Max Heaps using C++.

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## 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))
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