Easy to understand BSF traversal in Java.

Use BSF level traversal and find the 2nd smallest node value.

```
public int findSecondMinimumValue(TreeNode root) {
if (root == null) return -1;
int min = root.val;
int secondMin = Integer.MAX_VALUE;
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode curr = queue.poll();
if (curr.val > min && curr.val < secondMin) {
secondMin = curr.val;
}
if (curr.left != null) queue.offer(curr.left);
if (curr.right != null) queue.offer(curr.right);
}
}
return secondMin == Integer.MAX_VALUE ? -1 : secondMin;
}
```