UvaLive 4287 Proving Equivalences 强连通缩点

原题链接：https://icpcarchive.ecs.baylor.edu/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=2288

题意：

给你一个有向图，问你至少需要添加多少条边，使得整个图强连通。

题解：

就。。直接缩点，令缩点后入度为0的点有a个，出度为0的点有b个，答案就是max(a,b)

代码：

#include<iostream>
#include<cstring>
#include<vector>
#include<algorithm>
#include<stack>
#define MAX_N 20004
using namespace std;

vector<int> G[MAX_N];
int dfn[MAX_N],low[MAX_N],ind;
bool vis[MAX_N];
stack<int> st;
bool inStack[MAX_N];

int id[MAX_N],tot=0;
vector<int> newG[MAX_N];
vector<int> newrG[MAX_N];

int n,m;

void init() {
for (int i = 0; i <= n; i++) {
G[i].clear();
newG[i].clear();
newrG[i].clear();
}
memset(dfn, 0, sizeof(dfn));
memset(low, 0, sizeof(low));
ind = tot = 0;
memset(vis, 0, sizeof(vis));
while (st.size())st.pop();
memset(inStack, 0, sizeof(inStack));
memset(id, 0, sizeof(id));
}

void Tarjan(int u) {
dfn[u] = low[u] = ++ind;
st.push(u);
inStack[u] = 1;
vis[u] = 1;
for (int i = 0; i < G[u].size(); i++) {
int v = G[u][i];
if (!vis[v]) {
Tarjan(v);
low[u] = min(low[u], low[v]);
}
else if (inStack[v])
low[u] = min(low[u], dfn[v]);
}
if (low[u] == dfn[u]) {
tot++;
int t;
do {
t = st.top();
st.pop();
inStack[t] = 0;
id[t] = tot;
} while (t != u);
}
}

int main() {
int T;
cin.sync_with_stdio(false);
cin >> T;
while (T--) {
cin >> n >> m;
int ans = 0;
init();
for (int i = 0; i < m; i++) {
int u, v;
cin >> u >> v;
G[u].push_back(v);
}
for (int i = 1; i <= n; i++)
if (!vis[i])Tarjan(i);
for (int u = 1; u <= n; u++)
for (int i = 0; i < G[u].size(); i++)
if (id[u] != id[G[u][i]]) {
newG[id[u]].push_back(id[G[u][i]]);
newrG[id[G[u][i]]].push_back(id[u]);
}
if (tot == 1) {
cout << 0 << endl;
continue;
}
int a = 0, b = 0;
for (int u = 1; u <= tot; u++) {
if (newG[u].size() == 0)a++;
if (newrG[u].size() == 0)b++;
}
cout << max(a, b) << endl;
}
return 0;
}