每次找到最小叶子节点，输出它的相邻节点（叶子节点只有一个相邻节点），删除该叶子节点。循环对图做这个操作，直到只剩一个节点。最后一共要输出N-1个数。(finds the smallest leaf node at a time, and outputs its adjacent node (the leaf node has only one adjacent node) to delete the leaf node. The loop does this to the graph until there is only one node left. In the end, the total number of N-1 is output.)