/**
* isValidTree's recursive helper method.
*/
private void isValidHelper(int node, int[] counts) {
IntIterator edges = childEdgeRows(node);
int ncount = 0;
while ( edges.hasNext() ) {
// get next edge, increment count
int edge = edges.nextInt();
++ncount; ++counts[0];
// visit the next edge
int c = getAdjacentNode(edge, node);
isValidHelper(c, counts);
// check the counts