POJ 2762 going from U-V or from V to u? (Judging whether it is a weak link map)

Test instructions: Given a directional graph has M-bar one-way edge, to determine whether any two points can reach (a can to B or B can reach a or each other), that is to ask

Weak Unicom component.

Algorithm:

First, the strong connected components are obtained by shrinking points. Then re-build the map to determine whether the new diagram is a single chain, that is, can not fork, if the fork will exist in the case of unreachable.

How to tell if it is a single chain?

There is only one point in each of the 0 points, that is, there is only one point in each queue.

(O (╯-╰) o ..... It seems to be the second time to record the original point pair with a pair, and then save the vector of the pair forgot to empty led WA to WA! ）

#include <cstdio> #include <iostream> #include <cstring> #include <queue> #include <vector > #define MAXN 1010#define maxm 20010using namespace std;struct node{int to,next;} Edge[maxm],edge2[maxm];int Head[maxn],cnt,n;int Clk,top,s[maxn],scc,dfn[maxn],low[maxn],belong[maxn];bool Instack [Maxn],vis[maxn];int head2[maxn],cnt2,in[maxn];typedef pair<int,int> pii;vector<pii> xx;queue<int    > q;void Add (int x,int y) {edge[cnt].to = y;    Edge[cnt].next = Head[x]; HEAD[X] = cnt++;}    void add2 (int x,int y) {edge2[cnt2].to = y;    Edge2[cnt2].next = Head2[x]; HEAD2[X] = cnt2++;}    void Dfs (int x) {dfn[x] = low[x] = clk++;    s[top++] = x;    Instack[x] = true;        for (int i=head[x];i!=-1;i = edge[i].next) {int u = edge[i].to;            if (dfn[u]==-1) {dfs (U);        Low[x] = min (low[u],low[x]);        } else if (Instack[u]) {low[x] = min (Low[x],dfn[u]);  }} if (Low[x]==dfn[x]) {      int u;        scc++;            do {u = s[--top];            Instack[u]=false;        Belong[u] = SCC;    }while (u!=x);    }}void Tarjan () {memset (dfn,-1,sizeof (DFN));    memset (instack,0,sizeof (instack));    Memset (Belong,0,sizeof (belong));    CLK = top = SCC = 0;    for (int i=1;i<=n;i++) {if (dfn[i]==-1) DFS (i);    }}bool topo () {memset (vis,0,sizeof (VIS));    int c = 0;    while (!q.empty ()) Q.pop ();            for (int i=1;i<=scc;i++) {if (!in[i]) {C + +;        Q.push (i);    }} if (c>1) return false;        while (!q.empty ()) {int u = q.front ();        Q.pop ();        if (Vis[u]) continue;        Vis[u] = true;        c = 0;            for (int i=head2[u];i!=-1;i=edge2[i].next) {int v = edge2[i].to;            in[v]--;                if (!in[v]) {Q.push (v);            C + +; }} if (c>1) return false;    } return true;    int main () {int m,a,b,t;    scanf ("%d", &t);        while (t--) {scanf ("%d%d", &n,&m);        memset (head,-1,sizeof (head));        memset (In,0,sizeof (in));        Xx.clear ();        CNT = 0;            for (int i=1;i<=m;i++) {scanf ("%d%d", &a,&b);            Add (A, b);        Xx.push_back (Make_pair (A, b));        } Tarjan ();        memset (head2,-1,sizeof (head2));        Cnt2 = 0;            for (int i=0;i<xx.size (); i++) {int u = xx[i].first,v = Xx[i].second;                if (Belong[u]!=belong[v]) {ADD2 (belong[u],belong[v]);            in[belong[v]]++;        }} if (Topo ()) printf ("yes\n");    else printf ("no\n"); } return 0;}