Max-Heapify A Binary Tree

Question

This is one of the interview questions I recently came across.

Given the root address of a complete or almost complete binary tree, we have to write a function to convert the tree to a max-heap.

There are no arrays involved here. The tree is already constructed.

For e.g.,

              1   
         /         \
        2           5
      /   \       /   \ 
     3      4    6     7

can have any of the possible max heaps as the output--

              7   
         /         \
        3           6
      /   \       /   \ 
     2     1     4     5

or

              7   
         /         \
        4           6
      /   \       /   \ 
     2     3     1     5

etc...

I wrote a solution but using a combination of pre and post order traversals but that I guess runs in O(n^2). My code gives the following output.

              7   
         /         \
        3           6
      /   \       /   \ 
     1     2     4     5

I was looking for a better solution. Can somebody please help?

Edit :

My Code

void preorder(struct node* root)
{    
    if(root==NULL)return;
    max_heapify(root,NULL);
    preorder(root->left); 
    preorder(root->right);
}
void max_heapify(struct node* root,struct node* prev)
{
    if(root==NULL)
        return ;             
    max_heapify(root->left,root);
    max_heapify(root->right,root);
    if(prev!=NULL && root->data > prev->data)
    {
        swapper(root,prev);
    }     
}
void swapper(struct node* node1, struct node* node2)
{   
    int temp= node1->data;
    node1->data = node2->data;
    node2->data = temp;
}

Answer 1

I think this can be done in O(NlogN) time by the following procedure. http://www.cs.rit.edu/~rpj/courses/bic2/studios/studio1/studio121.html

Assume there is an element in the tree whose both left and right sub-trees are heaps.

          E
       H1   H2

This Tree formed by E, H1 and H2 can be heapified in logN time by making the element E swim down to its correct position.

Hence, we start building the heap bottom up. Goto the left-most sub-tree and convert it to a heap by trivial comparison. Do this for it's sibling as well. Then go up and convert it to heap.

Like-wise do this for every element.

EDIT: As mentioned in the comments, the complexity is actually O(N).