| Is It A Tree?
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.  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 45 6 0 08 1 7 3 6 2 8 9 7 57 4 7 8 7 6 0 03 8 6 8 6 45 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. Source |
题意：给出一系列的点，前者指向后者，求是否是一棵树。
思路：并查集思想，主要注意以下几组数据1: 0 0 空树是一棵树 2: 1 1 0 0 不是树 不能自己指向自己 3: 1 2 1 2 0 0 不是树....自己开始一直在这么WA 好郁闷 重复都不行呀~~5555 4: 1 2 2 3 4 5 0 0 不是树 森林不算是树(主要是注意自己) 5: 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 1 注意 一个节点在指向自己的父亲或祖先 都是错误的 即 9-->1 错 6: 1 2 2 1 0 0 也是错误的
#include#include #include using namespace std;const int N=10010;int father[N],b[N];int flag;int findSet(int x){ if(x!=father[x]){ father[x]=findSet(father[x]); } return father[x];}void Union(int a,int b){ if(b!=father[b]){ //如果结点编号不等于本身，说明有两条边指向它 flag=0; return ; } int x=findSet(a); int y=findSet(b); if(x==y){ //两个点的根结点相等 说明形成了环或本身指向本身 flag=0; return ; }else father[y]=x;}int main(){ //freopen("input.txt","r",stdin); int x,y; int cases=0; while(1){ flag=1; int cnt=0; for(int i=0;i