给定一棵有n个结点的二叉树，结点的编号为0~n-1。请你编写程序输出二叉树的前序遍历序列。 Input 输入的第一行是一个正整数t(1 < t < 20)，表示有t组测试用例。 对于每组测试用例，第一行是一个整数n(0 < n < 20)，表示二叉树结点个数。第二行是一个数r(0≤r≤n-1)，二叉树根结点的编号。后面有n-1行，表示二叉树n-1条边的信息。每行三个数a,b,c，三个数间由空格隔开，其中0≤a,b≤n-1且a≠b, c为0或1。a表示边的起点，

In computer science, a binary tree is a tree data structure in which each node has at most two children, which are referred to as the left child and the right child. A recursive definition using just set theory notions is that a (non-empty) binary tr