Check is a binary tree is binary search tree(improved)

gistfile1.java

static int lastNum=Integer.miniValue();
public boolean checkBST(TreeNode r){
if(r==null) return true;

if(!checkBST(r.left)) return false;

if(r.data<lastNum) return false;
lastNum=r.data;

if(!checkBST(r.right) return false;

return true;


}