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