输入一棵二叉树，第一行为2个整数N。代表二叉树有N（N<10）个节点（编号0 ~ N-1）。接下来N行，每行有两个数字，代表N个节点（从第0号节点开始到N-1号个节点）的左右孩子的编号（-1代表无）。（保证输入是符合一棵二叉树的）-Enter a binary tree, the first acts of two integers N. Represents binary tree has N (N < 10) nodes (number 0 ~ N-1). The next N lines, each line contains two numbers that represent the N nodes (node ​ ​ starts from No. 0 to No. N-1 nodes) of the number of children left (-1 means no). (Ensure that the input is in a binary tree)