BST Biggest Subtree within a range

Problem

Given a binary search tree t and a range (a,b), find number of nodes in the largest subtree or tree, whose nodes are in the range of (a,b). a, b are exclusive for the range.

Solution

public class BSTRange {
 
 private int max = 0;
 private boolean valid(int val, int a, int b){
  if(val > a && val < b) return true;
  return false;
 }

 private Integer postOrder(Tree t, int a, int b){
  if(t== null) return 0;

  Integer leftCnt = postOrder(t.left, a, b);
  Integer rightCnt = postOrder(t.right, a, b);

  if(leftCnt == null || rightCnt == null) return null;
  if(!valid(t.node, a, b)) return null;

  int cnt = 1 + leftCnt + rightCnt;
  if(max < cnt) max = cnt;
  return cnt; 
 }

 public int getMaxNodesInRange(Tree t, int a, int b){
  max =0;
  postOrder(t, a, b);
  return max;
 }

 public static void main(String[] args) {

  int[] preOrderData = {10,5,1,2,7,50,100};
  BSTFromPre ob = new BSTFromPre();
  Tree t = ob.reContructTree(preOrderData);
  TreeOperations.printTree(t);

  BSTRange ob1 = new BSTRange();
  System.out.println(ob1.getMaxNodesInRange(t, 0, 101));
 }
}