/* Given the binary tree node A[subtree_idx] whose children already
* satisfy the maxheap property, swap the node with its greater child
* until it is greater than both its children, so that the maxheap
/* Given the binary tree node A[subtree_idx] whose children already
* satisfy the maxheap property, swap the node with its greater child
* until it is greater than both its children, so that the maxheap