USACO Drainage Ditches

Code

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/*
ID: wcr19961
PROG: ditch
LANG: C++11
*/
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
using namespace std;
const int inf = 0x3f3f3f3f;
const int maxn = 210;
struct Edge {
int from, to, cap, flow;
Edge(int from, int to, int cap, int flow) : from(from), to(to), cap(cap), flow(flow) {}
};
bool operator < (const Edge& a, const Edge& b) {
return a.from < b.from || (a.from == b.from && a.to < b.to);
}
struct ISAP {
int n, m, s, t;
vector<Edge> edges;
vector<int> G[maxn]; // 邻接表,G[i][j]表示结点i的第j条边在e数组中的序号
bool vis[maxn]; // BFS使用
int d[maxn]; // 从起点到i的距离
int cur[maxn]; // 当前弧指针
int p[maxn]; // 可增广路上的上一条弧
int num[maxn]; // 距离标号计数
void AddEdge(int from, int to, int cap) {
edges.push_back(Edge(from, to, cap, 0));
edges.push_back(Edge(to, from, 0, 0));
m = edges.size();
G[from].push_back(m-2);
G[to].push_back(m-1);
}
bool BFS() {
memset(vis, 0, sizeof vis);
queue<int> Q;
Q.push(t);
vis[t] = 1;
d[t] = 0;
while(!Q.empty()) {
int x = Q.front(); Q.pop();
for(int i = 0; i < G[x].size(); i++) {
Edge& e = edges[G[x][i]^1];
if(!vis[e.from] && e.cap > e.flow) {
vis[e.from] = 1;
d[e.from] = d[x] + 1;
Q.push(e.from);
}
}
}
return vis[s];
}
void ClearAll(int n) {
this->n = n;
for(int i = 0; i < n; i++) G[i].clear();
edges.clear();
}
void ClearFlow() {
for(int i = 0; i < edges.size(); i++) edges[i].flow = 0;
}
int Augment() {
int x = t, a = inf;
while(x != s) {
Edge& e = edges[p[x]];
a = min(a, e.cap-e.flow);
x = edges[p[x]].from;
}
x = t;
while(x != s) {
edges[p[x]].flow += a;
edges[p[x]^1].flow -= a;
x = edges[p[x]].from;
}
return a;
}
int Maxflow(int s, int t) {
this->s = s; this->t = t;
int flow = 0;
BFS();
memset(num, 0, sizeof(num));
for(int i = 0; i < n; i++) num[d[i]]++;
int x = s;
memset(cur, 0, sizeof(cur));
while(d[s] < n) {
if(x == t) {
flow += Augment();
x = s;
}
int ok = 0;
for(int i = cur[x]; i < G[x].size(); i++) {
Edge& e = edges[G[x][i]];
if(e.cap > e.flow && d[x] == d[e.to] + 1) { // Advance
ok = 1;
p[e.to] = G[x][i];
cur[x] = i; // 注意
x = e.to;
break;
}
}
if(!ok) { // Retreat
int m = n-1; // 初值注意
for(int i = 0; i < G[x].size(); i++) {
Edge& e = edges[G[x][i]];
if(e.cap > e.flow && d[e.to] != -1) m = min(m, d[e.to]);
}
if(--num[d[x]] == 0) break;
num[d[x] = m+1]++;
cur[x] = 0; // 注意
if(x != s) x = edges[p[x]].from;
}
}
return flow;
}
vector<int> Mincut() { // call this after maxflow
BFS();
vector<int> ans;
for(int i = 0; i < edges.size(); i++) {
Edge& e = edges[i];
if(!vis[e.from] && vis[e.to] && e.cap > 0) ans.push_back(i);
}
return ans;
}
void Reduce() {
for(int i = 0; i < edges.size(); i++) edges[i].cap -= edges[i].flow;
}
} isap;
int main() {
freopen("ditch.in", "r", stdin);
freopen("ditch.out", "w", stdout);
int n, m, u, v, len;
scanf("%d%d", &m, &n);
isap.ClearAll(n + 1);
for (int i = 0; i < m; ++i) {
scanf("%d%d%d", &u, &v, &len);
isap.AddEdge(u, v, len);
}
printf("%d\n", isap.Maxflow(1, n));
return 0;
}

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