Question

What is wrong with this method ? it seems but I am not sure that the comparison of adjacent children in the tree does not take place.

I roughly traced the workings of this algorithm by hand and I think the idea is correct maybe something wrong with the implementation or I have no Idea how recursion works, the second helper (compare) method seems to be the issue

public static int MAX(BST B) {
  int m = ((Integer) B.root.data).intValue();
  return call(B.root, m);
}

public static int call(node current, int max) {   
  //first helper method gets the max from two different levels in the tree
  if(current == null)
    return -1;
  if(current.left == null && current.right == null)
    return max;
  else {
    if(((Integer) current.data).intValue()>max)
      max = ((Integer) current.data).intValue();
    return compare(call(current.left,max),call(current.right,max));
  }
}

//second helper method gets the max
static int compare(int m1, int m2) {
  if(m1>m2)
    return m1;
  else 
    return m2;
}

Answer 1

Since you are searching the entire tree, I’m going to assume that the structure is not properly ordered.

The bug is in your call function with:

 if(current.left==null&&current.right==null) return max;

Imagine you have a tree with a root with two leaf nodes (three nodes total). The root has value 3, right has value 2, and left has value 5. The algorithm should return 5, but your code will return 3. This is because you ignore the value of any leaf (a node with no “children”) with that line of code. So your code ignores the value 5, in this example, and returns max, which is 3.

You can fix this by returning compare(current.value, max) when left and right are null.