两题的核心思想都是判断是否成环，区别在于有向与无向，判断成环的条件就是find(a)=find(b),如果相等就成环，下面贴上AC代码
1 #include<cstdio> 2 #include<iostream> 3 using namespace std; 4 5 const int N=100005; 6 int pre[N]; 7 int mark[N]; 8 int flag; 9 10 void init() 11 { 12 for(int i=1; i<=N; i++) 13 { 14 mark[i]=0; 15 pre[i]=i; 16 } 17 } 18 int find(int x) 19 { 20 int r=x; 21 while(r!=pre[r]) 22 r=pre[r]; 23 int i=x,j; 24 while(i!=r) 25 { 26 j=pre[i]; 27 pre[i]=r; 28 i=j; 29 } 30 return r; 31 } 32 bool merge(int x,int y) 33 { 34 int fx=find(x); 35 int fy=find(y); 36 if(fx!=fy) 37 { 38 pre[fx]=fy; 39 return true; 40 } 41 else 42 { 43 return false; 44 } 45 } 46 int main() 47 { 48 int a,b; 49 while(~scanf("%d%d",&a,&b)&&(a!=-1&&b!=-1)) 50 { 51 init(); 52 flag=1; 53 int minn=99999999,maxn=-1; 54 if(a==0&&b==0) 55 { 56 printf("Yes "); 57 continue; 58 } 59 while(a||b) 60 { 61 if(a>maxn) maxn=a; 62 if(b>maxn) maxn=b; 63 if(b<minn) minn=b; 64 if(a<minn) minn=a; 65 mark[a]=1; 66 mark[b]=1; 67 if(merge(a,b)==false) 68 flag=0; 69 scanf("%d%d",&a,&b); 70 } 71 if(flag==0) 72 printf("No "); 73 else 74 { 75 int cnt=0; 76 for(int i=minn; i<=maxn; i++) 77 if(pre[i]==i&&mark[i]==1) 78 cnt++; 79 if(cnt==1) 80 printf("Yes "); 81 else 82 printf("No "); 83 } 84 } 85 return 0; 86 }