Uva11504-Dominos (Tarjan + contraction point)

Question Link

Q: The dominoes game gives some cards, and which card will be pushed down after the card falls down, and asks for the minimum number of cards to be pushed down so that all cards will go down.

Idea: The strongly connected component of a directed graph. After the point is reduced by Tarjan, find the number of points with a degree of 0, that is, the answer.

Code:

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;const int MAXN = 100100;const int MAXM = 100100;struct Edge{    int to, next;}edge[MAXM];int head[MAXN], tot;int Low[MAXN], DFN[MAXN], Stack[MAXN], Belong[MAXN];int Index, top;int scc;bool Instack[MAXN];int num[MAXN];int dg[MAXN];int n, m;void init() {    tot = 0;    memset(head, -1, sizeof(head));}void addedge(int u, int v) {    edge[tot].to = v;    edge[tot].next = head[u];    head[u] = tot++;}void Tarjan(int u) {    int v;    Low[u] = DFN[u] = ++Index;    Stack[top++] = u;    Instack[u] = true;    for (int i = head[u]; i != -1; i = edge[i].next) {        v = edge[i].to;                 if (!DFN[v]) {            Tarjan(v);             if (Low[u] > Low[v]) Low[u] = Low[v];        }         else if (Instack[v] && Low[u] > DFN[v])             Low[u] = DFN[v];    }    if (Low[u] == DFN[u]) {        scc++;         do {             v = Stack[--top];             Instack[v] = false;            Belong[v] = scc;            num[scc]++;        } while (v != u);     }}void solve() {    memset(Low, 0, sizeof(Low));    memset(DFN, 0, sizeof(DFN));    memset(num, 0, sizeof(num));    memset(Stack, 0, sizeof(Stack));    memset(Instack, false, sizeof(Instack));    Index = scc = top = 0;    for (int i = 1; i <= n; i++)         if (!DFN[i])            Tarjan(i);}int main() {    int cas;    scanf("%d", &cas);    while (cas--) {        init();        scanf("%d%d", &n, &m);                 int u, v;        for (int i = 0; i < m; i++) {            scanf("%d%d", &u, &v);             addedge(u, v);        }         solve();           memset(dg, 0, sizeof(dg));        for (int u = 1; u <= n; u++) {            for (int i = head[u]; i != -1; i = edge[i].next) {                int v = edge[i].to;                 if (Low[u] != Low[v])                     dg[Belong[v]]++;            }         }        int ans = 0;        for (int i = 1; i <= scc; i++)            if (dg[i] == 0)                ans++;        printf("%d\n", ans);    }    return 0;}

Uva11504-Dominos (Tarjan + contraction point)