USACO Drainage Ditches

Code

/*
 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;
}