POJ 2762 going from U-V or from V to u? (Strong unicom, topological sort)

http://poj.org/problem?id=2762

Going from U-V or from V to u? Time Limit: 2000MS Memory Limit: 65536K Total Submissions: 14573 Accepted: 3849 Description In order to make their sons brave, Jiajia and wind take them to a big cave. The cave has n rooms, and one-way corridors connecting some rooms. Each time, wind choose the rooms x and Y, and ask one of the their little sons go from one to the other. The son can either go from X-to-y, or from Y to X. Wind promised that her tasks is all possible, but she actually doesn ' t Know how to decide if a task is possible. To make hers life easier, Jiajia decided to choose a cave in which every pair of rooms are a possible task. Given a cave, can you tell Jiajia whether wind can randomly choose both rooms without worrying about anything? Input The first line contains a single integer T, the number of test cases. and followed T cases. The first line is contains integers n, m (0 < N < 1001,m < 6000), the number of rooms and corridors In the cave. The next m lines each contains to integers u and V, indicating that there is a corridor connecting the class U and the Class V dire ctly. Output The output should contain T lines. Write ' Yes ' if the cave have the property stated above, or ' No ' otherwise. Sample Input 13 31 22) 33 1 Sample Output Yes Source POJ Monthly--2006.02.26,zgl & TWB

Test instructions

Given a direction graph, determine if any two-point u,v can be reached from U to V or from V to U.

Analysis:

Determine the single connectivity to the graph. First, to get a dag, if the DAG is a single chain, it is clear that it is possible. How can I tell if a dag is a single chain? Just determine if the topological order is unique.

/* * * author:fcbruce <[email protected]> * * time:tue 11:37:19 AM CST */#include <cstdio& GT, #include <iostream> #include <sstream> #include <cstdlib> #include <algorithm> #include < ctime> #include <cctype> #include <cmath> #include <string> #include <cstring> #include < stack> #include <queue> #include <list> #include <vector> #include <map> #include <set> #define SQR (x) ((x) * (x)) #define LL long long#define ITN int#define INF 0x3f3f3f3f#define PI 3.1415926535897932384626#defi NE eps 1e-10#ifdef _win32 #define LLD "%i64d" #else #define LLD "%lld" #endif # define MAXM 8964#define MAXN 1007using names Pace std;int N,m;int fir[maxn];int u[maxm],v[maxm],nex[maxm];int e_max;int pre[maxn],low[maxn],sccno[maxn];int st[  Maxn],top;int scc_cnt,dfs_clock;int deg[maxn];inline void add_edge (int _u,int _v) {int e=e_max++;  U[e]=_u;v[e]=_v; Nex[e]=fir[u[e]];fir[u[e]]=e;} void Tarjan_dfs (int s) {pre[S]=low[s]=++dfs_clock;  St[++top]=s;    for (int e=fir[s];~e;e=nex[e]) {int t=v[e];      if (pre[t]==0) {Tarjan_dfs (t);    Low[s]=min (Low[s],low[t]);    } else {if (sccno[t]==0) low[s]=min (low[s],pre[t]);    }} if (Low[s]==pre[s]) {scc_cnt++;    for (;;)      {int x=st[top--];      sccno[x]=scc_cnt;    if (x==s) break;  }}}void FIND_SCC () {scc_cnt=dfs_clock=0;  Top=-1;  memset (sccno,0,sizeof sccno);  memset (pre,0,sizeof Pre); for (int i=1;i<=n;i++) if (pre[i]==0) Tarjan_dfs (i);}  BOOL Unique_toposort () {top=-1;  for (int i=1;i<=scc_cnt;i++) {if (deg[i]==0) st[++top]=i;    } for (int i=0;i<scc_cnt;i++) {if (Top==1 | | | top==-1) return FALSE;    int x=st[top--];      for (int e=fir[x];~e;e=nex[e]) {deg[v[e]]--;    if (deg[v[e]]==0) st[++top]=v[e]; }} return true;  int main () {#ifdef fcbruce freopen ("/home/fcbruce/code/t", "R", stdin), #endif//fcbruce int t_t;  scanf ("%d", &t_t); while (t_t--) {SCANF ("%d%d", &n,&m);    e_max=0;    memset (fir,-1,sizeof fir);      for (int i=0,u,v;i<m;i++) {scanf ("%d%d", &u,&v);    Add_edge (U,V);    } FIND_SCC ();    int Temp=e_max;    e_max=0;    memset (fir,-1,sizeof fir);    memset (deg,0,sizeof deg);      for (int e=0;e<temp;e++) {if (Sccno[u[e]]==sccno[v[e]]) continue;      deg[sccno[v[e]]]++;    Add_edge (Sccno[u[e]],sccno[v[e]);    } if (Unique_toposort ()) puts ("Yes");      Else puts ("No"); } return 0;}

POJ 2762 going from U-V or from V to u? (Strong unicom, topological sort)