LightOJ 1210 - Efficient Traffic System【强连通图】

题目链接：http://www.lightoj.com/volume_showproblem.php?problem=1210

题意：
加多少条边，使得整个图变得强连通。
使用Tarjan进行缩点，得到一个SCC图、
这个图有多少个入度为0的，多少个出度为0的。
假设有n个入度为0，m个出度为0
那么答案是max(n,m)

代码：

#include <stdio.h>#include <iostream>#include <math.h>#include <stdlib.h>#include <ctype.h>#include <algorithm>#include <vector>#include <string.h>#include <string>#include <queue>#include <stack>#include <set>#include <map>#include <sstream>#include <time.h>using namespace std;const int MAXN = 200100;const int MAXM = 500100;struct Edge{    int to, next;}edge[MAXM];int head[MAXM], tot;int Low[MAXN], Dfn[MAXN], Stack[MAXN], Belong[MAXN];//Belong的值为 1 ~ sccint Index, top;int scc;//强连通个数bool Instack[MAXN];int num[MAXN];//各个强连通包含的点的个数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);    }}int in[MAXN], out[MAXN];int cases = 1;void solve(int N){    memset(Dfn, 0, sizeof(Dfn));    memset(Instack, false, sizeof(Instack));    memset(num, 0, sizeof(num));    Index = scc = top = 0;    for (int i = 1; i <= N; i++)    {        if (!Dfn[i])            Tarjan(i);    }    printf("Case %d: ",cases++);    if (scc == 1)    {        printf("0\n");        return;    }    for (int i = 1; i <= scc; i++)        in[i] = out[i] = 0;    for (int u = 1; u <= N; u++)    {        for (int i = head[u]; i != -1; i = edge[i].next)        {            int v = edge[i].to;            if (Belong[u] != Belong[v])            {                in[Belong[v]]++;                out[Belong[u]]++;            }        }    }    int ans1 = 0, ans2 = 0;    for (int i = 1; i <= scc; i++)    {        if (in[i] == 0) ans1++;        if (out[i] == 0) ans2++;    }    printf("%d\n", max(ans1, ans2));}void init(){    tot = 0;    memset(head, -1, sizeof(head));}int main(){    int n, t, m;    int u, v;    scanf("%d", &t);    while (t--)    {        init();        scanf("%d%d", &n, &m);        for (int i = 1; i <= m; i++)        {            scanf("%d%d", &u, &v);            {                addedge(u, v);            }        }        solve(n);    }    return 0;}