CCF-CSP题解 201509-4 高速公路

有点忧愁。(CSP)也考(Tarjan)缩点的嘛。

原理咱也不明白，咱也不敢学，找到模板就是抄。

#include<bits/stdc++.h>
const int maxn = 10000;
const int maxm = 100000;

using namespace std;

int to[maxm + 10];
int nex[maxm + 10];
int head[maxn + 10], cnt = 0;

void addEdge(int a, int b)
{
    to[cnt] = b; nex[cnt] = head[a]; head[a] = cnt++;
}

int low[maxn + 10], dfn[maxn + 10], sta[maxn + 10], belong[maxn + 10];
int index, top;
int scc;
bool inStack[maxn + 10];
int num[maxn + 10];

void Tarjan(int u)
{
    int v;
    low[u] = dfn[u] = ++index;
    sta[top++] = u;
    inStack[u] = true;
    for (int i = head[u]; i != -1; i = nex[i])
    {
        v = to[i];
        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 = sta[--top];
            inStack[v] = false;
            belong[v] = scc;
            num[scc]++;
        } while (v != u);
    }
}

int main()
{
    int n, m;
    scanf("%d%d", &n, &m);

    memset(head, -1, sizeof(head));
    for (int i = 1, a, b; i <= m; i++)
    {
        scanf("%d%d", &a, &b);
        addEdge(a, b);
    }

    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);
    }

    int ans = 0;
    for (int i = 1; i <= scc; i++)
    {
        ans += num[i] * (num[i] - 1) / 2;
    }
    printf("%d
", ans);

    return 0;
}