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| #include <iostream>
#include <cstdio>
#include <stack>
#define maxn 400010
#define maxm 400010
using namespace std;
int n, m;
struct Edge {
int to, next, f;
} e[maxm * 2]; int c1, head[maxn];
inline void add_edge(int u, int v) {
e[c1].to = v; e[c1].next = head[u]; head[u] = c1++;
}
int dfn[maxn], low[maxn], rt, mx; stack<int> S;
void tarjan(int u, int fa) {
static int cnt = 0;
dfn[u] = low[u] = ++cnt; S.push(u);
for (int i = head[u]; ~i; i = e[i].next) {
int v = e[i].to, &f1 = e[i].f, &f2 = e[i ^ 1].f; if (v == fa) continue;
if (!dfn[v]) {
tarjan(v, u), low[u] = min(low[u], low[v]);
if (low[v] > dfn[u]) f1 = f2 = 1;
}
else low[u] = min(low[u], dfn[v]);
}
if (low[u] == dfn[u]) {
int sum = 0, t;
do {
t = S.top(); S.pop();
++sum;
} while (t != u);
if (sum > mx) mx = sum, rt = u;
}
}
bool vis[maxn], ins[maxn]; int au[maxm], av[maxm];
void dfs(int u, int fa) {
vis[u] = 1; ins[u] = 1;
for (int i = head[u]; ~i; i = e[i].next) {
int v = e[i].to, f = e[i].f; if (v == fa) continue;
if (vis[v]) {
if (ins[v]) au[i / 2 + 1] = u, av[i / 2 + 1] = v;
continue;
}
if (f) au[i / 2 + 1] = v, av[i / 2 + 1] = u;
else au[i / 2 + 1] = u, av[i / 2 + 1] = v;
dfs(v, u);
} ins[u] = 0;
}
int main() { fill(head, head + maxn, -1);
ios::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
cin >> n >> m;
for (int i = 1; i <= m; ++i) {
int x, y; cin >> x >> y;
add_edge(x, y); add_edge(y, x);
} tarjan(1, 0); dfs(rt, 0);
cout << mx << "\n";
for (int i = 1; i <= m; ++i) cout << au[i] << " " << av[i] << "\n";
return 0;
}
|