Is It A Tree?（普通的思维逻辑）

Is It A Tree?
Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 1236 Accepted Submission(s): 389

Problem Description
A tree is a well-known data structure that is either empty (null, void, nothing) or is a set of one or more nodes connected by directed edges between nodes satisfying the following properties.
There is exactly one node, called the root, to which no directed edges point.

Every node except the root has exactly one edge pointing to it.

There is a unique sequence of directed edges from the root to each node.

For example, consider the illustrations below, in which nodes are represented by circles and edges are represented by lines with arrowheads. The first two of these are trees, but the last is not.

In this problem you will be given several descriptions of collections of nodes connected by directed edges. For each of these you are to determine if the collection satisfies the definition of a tree or not.

Input
The input will consist of a sequence of descriptions (test cases) followed by a pair of negative integers. Each test case will consist of a sequence of edge descriptions followed by a pair of zeroes Each edge description will consist of a pair of integers; the first integer identifies the node from which the edge begins, and the second integer identifies the node to which the edge is directed. Node numbers will always be greater than zero.

Output

        For each test case display the line ``Case k is a tree.\\\\\\\" or the line ``Case k is not a tree.\\\\\\\", where k corresponds to the test case number (they are sequentially numbered starting with 1).

Sample Input
6 8 5 3 5 2 6 4
5 6 0 0
8 1 7 3 6 2 8 9 7 5
7 4 7 8 7 6 0 0
3 8 6 8 6 4
5 3 5 6 5 2 0 0
-1 -1

Sample Output
Case 1 is a tree.
Case 2 is a tree.
Case 3 is not a tree.

/*一开始我是想通过并查集来做的可是写到程序读入时以后我突然发现可以不用通过并查集来做 就是一个简单思维逻辑 不是树的情况下主要有三种情况 一种是某一节点的入度大于等于2 或者这个图存在多个(大于等于2)根节点即出度为一 或者是 不存在根节点 即存在一个环
*/

#include<iostream>#include<algorithm>#include<string>#include<string.h>#include<stdio.h>#include<set>using namespace std;int ru[3000];int chu[3000];int main(){    int st,ed;    int cnt =1;    while(cin>>st>>ed&&(st + ed)>=0)    {        int flag =0;        memset(ru,0,sizeof(ru));        memset(chu,-1,sizeof(chu));        ru[ed]++;        chu[st] = 1;        chu[ed] = 0;        while(cin>>st>>ed&&(st + ed))        {            if(flag) continue;            ru[ed]++;            if(chu[st]==0&&chu[ed]==1)                flag = 1;            chu[st] = (chu[st]==-1?1:chu[st]);            chu[ed] = 0;        }        sort(ru,ru+3000);        sort(chu,chu+3000);        cout<<"Case "<<cnt++<<" ";        if(ru[2999]>=2||flag||chu[2998])        {            cout<<"is not a tree."<<endl;        }        else            cout<<"is a tree."<<endl;    }    return 0;}