Is It A Tree

题目描述

By definition, a tree is a collection of nodes. The collection can be empty; otherwise, a tree consists of a distinguished node $r$, called the root; and zero or more nonempty (sub)trees, each of whose roots are connected by a directed edge from $r$. In other words, a tree is a spacial case of a directed graph. Given a directed graph, your job is to tell whether or not it is a tree.

输入格式

Each input file contains one test case, which starts from a line containing a positive integer $n (≤10^4)$, the number of nodes. Hence we assume that all the nodes are numbered from 1 to n. Then several lines follow, each describes a directed edge in the format

source destination

where both are the nodes' indices. It is guaranteed that there are no more than $10^4$ edges, and that source is never the same as destination. The input ends with source being zero, and that line must not be processed. Note: duplicated edges are counted as ONE edge (as shown by the first sample).

输出格式

If the given graph is a tree, out put in a line yes root, where root is the index of the root. Or if not, output no k where k is the number of nodes with 0 indegree.

样例 #1

样例输入 #1

7
2 1
2 1
4 3
4 2
6 5
6 4
6 7
0

样例输出 #1

yes 6