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| #include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
#define MAXN 1000010
#define INF 2000000000
struct Edge {
int v, nx, w;
}e[MAXN << 2], _e[MAXN << 2];
struct E {
int x, y;
}a[MAXN << 2];
int _head[MAXN], head[MAXN], ecnt = 1, _ecnt, n, m, col[MAXN], dep[MAXN], cur[MAXN];
int dfn[MAXN], low[MAXN], tim, st[MAXN], top, num, in[MAXN], r, k, cnt;
bool vis[MAXN];
bool cmp(E x, E y) {
return x.x < y.x || (x.x == y.x && x.y < y.y);
}
int min(int a, int b) {
if (a < b) return a;
else return b;
}
void add(int f, int t, int w) {
e[++ecnt] = (Edge) {t, head[f], w};
head[f] = ecnt;
e[++ecnt] = (Edge) {f, head[t], 0};
head[t] = ecnt;
}
void _add(int f, int t) {
_e[++_ecnt] = (Edge) {t, _head[f], 0};
_head[f] = _ecnt;
}
bool bfs(int s, int t) {
memset(dep, 0x7f, sizeof(dep));
dep[s] = 0;
for (int i = 1; i <= k; i++) {
cur[i] = head[i];
}
std::queue <int> q;
q.push(s);
while (!q.empty()) {
int u = q.front();
q.pop();
for (int i = head[u]; i; i = e[i].nx) {
int to = e[i].v;
if (dep[to] > INF && e[i].w) {
dep[to] = dep[u] + 1;
q.push(to);
}
}
}
return dep[t] < INF;
}
int dfs(int s, int t, int l) {
if (!l || s == t) {
return l;
}
int fl = 0, f;
for (int i = cur[s]; i; i = e[i].nx) {
cur[s] = i;
int to = e[i].v;
if (dep[to] == dep[s] + 1 && (f = dfs(to, t, min(l, e[i].w)))) {
fl += f;
l -= f;
e[i].w -= f;
e[i ^ 1].w += f;
if (!l) break;
}
}
return fl;
}
int Dinic(int s, int t) {
int maxf = 0;
while (bfs(s, t)) {
maxf += dfs(s, t, INF);
}
return maxf;
}
void tarjan(int u) {
dfn[u] = low[u] = ++tim;
st[++top] = u;
vis[u] = 1;
for (int i = _head[u]; i; i = _e[i].nx) {
int v = _e[i].v;
if (!dfn[v]) {
tarjan(v);
low[u] = min(low[u], low[v]);
}
else if (vis[v]) low[u] = min(low[u], dfn[v]);
}
if (dfn[u] == low[u]) {
num++;
int v;
do {
v = st[top--];
vis[v] = 0;
in[v] = num;
} while (u != v);
}
}
void dfscol(int x, int cr) {
col[x] = cr;
for (int i = _head[x]; i; i = _e[i].nx) {
int to = _e[i].v;
if (!col[to]) {
dfscol(to, 3 - cr);
}
}
}
int main() {
scanf("%d%d", &n, &m);
r = n + 1;
k = r + 1;
for (int i = 1; i <= m; i++) {
scanf("%d%d", &a[i].x, &a[i].y);
_add(a[i].x, a[i].y);
_add(a[i].y, a[i].x);
}
for (int i = 1; i <= n; i++) {
if (!col[i]) dfscol(i, 1);
}
for (int i = 1; i <= n; i++) {
if (col[i] == 1) {
add(r, i, 1);
}
if (col[i] == 2) {
add(i, k, 1);
}
}
memset(_head, 0, sizeof(_head));
_ecnt = 0;
memset(_e, 0, sizeof(_e));
for (int i = 1; i <= m; i++) {
if (col[a[i].x] == 1) {
add(a[i].x, a[i].y, 1);
}
if (col[a[i].y] == 1) {
add(a[i].y, a[i].x, 1);
}
}
int tot = Dinic(r, k);
for (int i = head[r]; i; i = e[i].nx) {
int to = e[i].v;
if (!e[i].w) {
_add(to, r);
}
if (e[i].w) {
_add(r, to);
}
for (int j = head[to]; j; j = e[j].nx) {
int v = e[j].v;
if (v == r) continue;
if (e[j].w) {
_add(to, v);
}
else {
_add(v, to);
}
}
}
for (int i = head[k]; i; i = e[i].nx) {
int to = e[i].v;
if (e[i].w) {
_add(k, to);
}
if (!e[i].w) {
_add(to, k);
}
}
for (int i = 1; i <= n + 2; i++) {
if (!dfn[i]) tarjan(i);
}
memset(a, 0, sizeof(a));
for (int i = 1; i <= n; i++) {
if (col[i] == 1) {
for (int j = head[i]; j; j = e[j].nx) {
int v = e[j].v;
if (v == r) continue;
if (!e[j].w) {
a[++cnt].x = i;
a[cnt].y = v;
if (a[cnt].x > a[cnt].y) std::swap(a[cnt].x, a[cnt].y);
}
}
}
}
tot = 0;
for (int i = 1; i <= cnt; i++) {
if (in[a[i].x] != in[a[i].y]) {
if (a[i].x > a[i].y) std::swap(a[i].x, a[i].y);
tot++;
}
}
std::sort(a + 1, a + 1 + cnt, cmp);
printf("%d\n", tot);
for (int i = 1; i <= cnt; i++) {
if (in[a[i].x] != in[a[i].y]) {
printf("%d %d\n", a[i].x, a[i].y);
}
}
return 0;
}
|
