Is it a tree?

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题目：

http://poj.org/problem?id=1308

大意：

给你一堆树枝，让你判断最后能不能构成一棵树。

思路：根据树的定义判断即可

树：

1.入度只能为1或者0（根结点）

2.根结点只有一个

3.出度可以有多个

代码如下：

#include<iostream>#include<cstdio>#include<cstring>#include<string>#include<algorithm>using namespace std;#define MAXN 10010#pragma comment(linker, "/STACK:102400000,102400000")int pre[10010], son[10010], visit[10010];int unionsearch(int root) //查找根结点+路径压缩（递归容易破栈）{int root2;root2 = root;while(root != pre[root])root = pre[root];while(root2 != root){int temp = pre[root2];pre[root2] = root;root2 = temp;}return root;}bool cmp(){int root = 0;for(int i = 0; i < MAXN; ++i) //累计根结点的子节点个数if(visit[i])++son[unionsearch(i)];for(int i = 0; i < MAXN; ++i) //累计根结点的个数if(son[i] > 1)root++;if(root > 1) //判断为树或者为森林return false;else return true;}int main(){//freopen("Input.txt", "r", stdin);int start, end, x, y, flag, T = 0;while(true){flag = 0;for(int i = 0; i < MAXN; ++i)pre[i] = i;memset(son, 0, sizeof(son));memset(visit, 0, sizeof(visit));while(scanf("%d%d", &start, &end) && start && end){if(start == -1 && end == -1) return 0;x = unionsearch(start);y = unionsearch(end);visit[start] = 1; // 树枝标记，忘了。调试了N久。visit[end] = 1;if(x == y || end != y) //x == y为环，end == y，入度为2flag = 1;elsepre[end] = start;}if(flag)printf("Case %d is not a tree.\n", ++T);else{if(cmp())printf("Case %d is a tree.\n", ++T);elseprintf("Case %d is not a tree.\n", ++T);}}return 0;}

改进后的程序（2011/10/25）

 #include<iostream>#include<cstdio>#include<cstring>#include<string>#include<algorithm>using namespace std;#define MAXN 10010int pre[10010], visit[10010];int unionsearch(int root) //查找根结点+路径压缩{int root2;root2 = root;while(root != pre[root])root = pre[root];while(root2 != root){int temp = pre[root2];pre[root2] = root;root2 = temp;}return root;}bool cmp(){int root = 0;for(int i = 0; i < MAXN; ++i) //累计根结点的子节点个数if(visit[i] && pre[i] == i)root++;if(root > 1) //判断为树或者为森林return false;else return true;}int main(){//freopen("Input.txt", "r", stdin);int start, end, x, y, flag, T = 0;while(true){flag = 0;for(int i = 0; i < MAXN; ++i)pre[i] = i;memset(visit, 0, sizeof(visit));while(scanf("%d%d", &start, &end) && start && end){if(start == -1 && end == -1) return 0;x = unionsearch(start);y = unionsearch(end);visit[start] = 1;visit[end] = 1;if(x == y || end != y)flag = 1;elsepre[end] = start;}if(flag)printf("Case %d is not a tree.\n", ++T);else{if(cmp())printf("Case %d is a tree.\n", ++T);elseprintf("Case %d is not a tree.\n", ++T);}}return 0;}