POJ1038 Is It A Tree?

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思考：首先对这道题目很无语，没有数据量。

注意一下几点：1. 空树2.仅有一个树根3森林的情况。

如果是一棵树则N个点，N-1条边（如果连通）如果是森林则不连通。

同时满足连通且N个顶点有N-1条边。这个代码好烂！

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <vector>using namespace std;const int maxn = 100000; // 10^6 Memory Limit Exceededvector<int> v[maxn+10];bool vis[maxn+5];int edge_num;int node_num;int travel_node;void dfs(int x) {if(!vis[x]) {      travel_node++;      vis[x] = true;    } else return;for(int i = 0; i < (int)v[x].size(); i++) {dfs(v[x][i]);}}void init(){    edge_num = 0;    node_num = 0;    for(int i = 0; i < maxn; i++) v[i].clear();}int main(){bool ok = true;int index = 0;int start; // dfs from this point.while(ok) {init();start = 0;int x, y;int a = 0, b = 0;while(scanf("%d%d", &x, &y) == 2 && (x || y)) {if(x < 0 && y < 0) {ok = false; break;}v[x].push_back(y);v[y].push_back(x);edge_num++;if(!start) start = x;if(a==0 && b==0) {                a = x;                b = y;}}if(!ok) break;if(x==0 && y==0 && a == 0 && b == 0) { //0 0            printf("Case %d is a tree.\n", ++index);            continue;        }for(int i = 1; i < maxn; i++) {if(v[i].size()) node_num++;}if(edge_num+1 == node_num); // exist ring.else {printf("Case %d is not a tree.\n", ++index);continue;}// whether the tree is connectedmemset(vis, false, sizeof(vis));travel_node = 0;dfs(start);if(travel_node == node_num) {printf("Case %d is a tree.\n", ++index);} else {printf("Case %d is not a tree.\n", ++index);}}return 0;}

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