gfg - basic heaps problem

class GfG{
    //1. build heap
    //2. heap sort
    //http://www.cs.toronto.edu/~krueger/cscB63h/w07/lectures/tut02.txt
    
    
    void buildHeap(int arr[], int n){
        //base case:
        if(n<1) return;
        
        //build heap: do heapify on all non-leaf nodes
         for (int i = n/2 -1; i >= 0; i--)
            heapify(arr, n, i);
    
        
        //heap sort:
        //for getting in-place ascemding order:
        //  - make max heap
        //  - swap max at root with last leaf
        //  - shorten arr and max heap again
        for(int i=n-1; i>=0; i--){
            int tmp = arr[0];
            arr[0]=arr[i];
            arr[i]=tmp;
            
            heapify(arr,i,0);
        }
    }
 
    //max heap
    void heapify(int arr[], int n, int i){
      int s = i; 
        int l = 2*i + 1;  
        int r = 2*i + 2;  
        
        //base case:
        if(l>n-1 && r>n-1) return;
        
        if(l<n && arr[l]>arr[s]) s=l;
        if(r<n && arr[r]>arr[s]) s=r;
        
        if(s==i) return;
        
        int tmp = arr[i];
        arr[i] = arr[s];
        arr[s] = tmp;
        
        heapify(arr,n,s);
    }
}