Do BFS from the root. Let the root be at column 0. In the BFS, keep in the queue the node and its column.

When you traverse a node, store its value in the column index. For example, the root's value should be stored at index 0.

If the node has a left node, it column should be col - 1. Similarly, if the node has a right node, its column should be col + 1.

At the end, check the minimum and maximum col and output their values.