NKNET - Mạng truyền tin
Tác giả: ladpro98
Ngôn ngữ: C++
#include <bits/stdc++.h>
#define ii pair<int, int>
#define iii pair<int, ii>
#define X first
#define Y second
const int N = 111;
const int oo = trunc(1e9);
const int V = N * N;
using namespace std;
vector<iii> e;
vector<int> a[N];
int c[N][N], f[N][N], T[N], Q[N];
int m, n, s, t, res, ans;
void Enter() {
//freopen("NKNET.in", "r", stdin);
scanf("%d %d\n", &n, &m);
int u, v, c;
for(int i=1; i<=m; i++) {
scanf("%d %d %d\n", &u, &v, &c);
e.push_back(iii(c, ii(u, v)));
a[u].push_back(v);
a[v].push_back(u);
}
scanf("%d %d", &s, &t);
}
void Init(int lim) {
//initialize network
int i, j, u, v;
for(i=1; i<=n; i++) {
a[i].clear();
for(j=1; j<=n; j++) {
f[i][j] = 0;
c[i][j] = V;
}
}
for(i=0; i<m; i++)
if (e[i].X > lim) {
u = e[i].Y.X;
v = e[i].Y.Y;
a[u].push_back(v);
a[v].push_back(u);
}
}
void Init2(int lim) {
int i, j, u, v;
for(i=1; i<=n; i++) {
a[i].clear();
for(j=1; j<=n; j++) {
f[i][j] = 0;
c[i][j] = V;
}
}
for(i=0; i<m; i++) {
u = e[i].Y.X;
v = e[i].Y.Y;
a[u].push_back(v);
a[v].push_back(u);
if (e[i].X <= lim)
c[u][v] = c[v][u] = 1;
}
}
bool FindPath() {
int l=1, r=1, u, v, i;
Q[1] = s;
for(i=1; i<=n; i++) T[i] = 0;
while (l <= r) {
u = Q[l++];
for(i=0; i<a[u].size(); i++) {
v = a[u][i];
if (T[v] == 0 && c[u][v] > f[u][v]) {
Q[++r] = v;
T[v] = u;
if (v == t) return true;
}
}
}
return false;
}
void IncFlow() {
int u, v, delta = oo;
v = t;
while (v != s) {
u = T[v];
delta = min(delta, c[u][v] - f[u][v]);
v = u;
}
v = t;
while (v != s) {
u = T[v];
f[u][v] += delta;
f[v][u] -= delta;
v = u;
}
}
int maxFlow() {
while (FindPath()) IncFlow();
int flow = 0;
for(int i=0; i<a[s].size(); i++)
flow += f[s][a[s][i]];
return flow;
}
int main()
{
Enter();
//BinarySearch the answer
int l = 0, r = V, mid;
while (l <= r) {
mid = (l + r) / 2;
Init(mid);
if (FindPath()) {
l = mid + 1;
}
else {
res = mid;
r = mid - 1;
}
}
Init2(res);
ans = maxFlow();
printf("%d\n", ans);
for(int i=1; i<=n; i++)
if (i == s || T[i] > 0) {
for(int j=0; j<a[i].size(); j++) {
int v = a[i][j];
if (T[v] == 0) printf("%d %d\n", i, v);
}
}
return 0;
}