Is It A Tree?
Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 6961 Accepted Submission(s): 1619
【题目链接】http://acm.hdu.edu.cn/showproblem.php?pid=1325
这题如果用并查集做的话，最后判断的时候还是要看一个结点的入度是否为大于1（纯并查集的情况下），搜了几份代码，有些是去判断是否有环的，这些都不如直接用树的充分条件去判断：一个节点入度为0，其余节点入读为1，V个点只有V-1条边，题目坑你的地方大概有两点：没节点也是树，结束时两个数是负数而不是两个-1
1 #include <cstdio> 2 #include <cstring> 3 #define SIZE 2013 4 using namespace std; 5 int father[SIZE]; 6 bool exit[SIZE]; 7 int find(int f) 8 { 9 return father[f] = f == father[f] ? f : find(father[f]); 10 } 11 12 int main() 13 { 14 #ifndef ONLINE_JUDGE 15 freopen("input.txt", "r", stdin); 16 #endif 17 int n, m, T = 0, max = 0, nv = 0, ne = 0; 18 bool flag = true; 19 memset(father, 0, sizeof(father)); 20 memset(exit, false, sizeof(exit)); 21 while(scanf("%d%d", &n, &m) != EOF) 22 { 23 if(n < 0 && m < 0) break; 24 else if(n+m == 0) 25 { 26 if(ne == 0 && nv == 0) 27 { 28 printf("Case %d is a tree. ", ++T); 29 continue; 30 } 31 if(ne == nv-1) 32 { 33 int cnt = 0, zero = 0; 34 for(int i = 0; i <= max; ++i) 35 if(exit[i]) 36 { 37 if(!father[i]) zero++; 38 else if(father[i] == 1) cnt++; 39 } 40 if(cnt == ne && zero == 1) 41 printf("Case %d is a tree. ", ++T); 42 else 43 printf("Case %d is not a tree. ", ++T); 44 } 45 else printf("Case %d is not a tree. ", ++T); 46 47 flag = true; 48 memset(exit, false, sizeof(exit)); 49 memset(father, 0, sizeof(father)); 50 ne = nv = 0; 51 max = 0; 52 } 53 else 54 { 55 max = max > n ? (max > m ? max : m) : (n > m ? n : m); 56 if(!exit[n]) 57 { 58 exit[n] = true; 59 nv++; 60 } 61 if(!exit[m]) 62 { 63 exit[m] = true; 64 nv++; 65 } 66 ne++; 67 father[m]++; 68 } 69 } 70 return 0; 71 }