HDU 4280：Island Transport（ISAP模板题）

http://acm.hdu.edu.cn/showproblem.php?pid=4280

题意：在最西边的点走到最东边的点最大容量。

思路：ISAP模板题，Dinic过不了。

 #include <cstdio>
 #include <algorithm>
 #include <iostream>
 #include <cstring>
 #include <string>
 #include <cmath>
 #include <queue>
 #include <vector>
 #include <map>
 #include <set>
 using namespace std;
 #define INF 0x3f3f3f3f
 #define N 100010
 typedef long long LL;
 struct Edge {
     int u, v;
     LL cap;
     Edge () {}
     Edge (int u, int v, LL cap) : u(u), v(v), cap(cap) {}
 }edge[N*];
 vector<int> G[N];
 int dis[N], cur[N], num[N], pre[N], tot, S, T;

 void Add(int u, int v, int cap) {
     edge[tot] = Edge(u, v, cap);
     G[u].push_back(tot++);
     edge[tot] = Edge(v, u, cap);
     G[v].push_back(tot++);
 }

 int BFS() { // 逆向BFS
     memset(dis, -, sizeof(dis));
     queue<int> que; que.push(T);
     dis[T] = ;
     while(!que.empty()) {
         int u = que.front(); que.pop();
         for(int i = ; i < G[u].size(); i++) {
             Edge &e = edge[G[u][i]];
             if(dis[e.v] == -) {
                 dis[e.v] = dis[u] + ;
                 que.push(e.v);
             }
         }
     }
     return ~dis[S];
 }

 int DFS() { // 通过pre数组回溯更新流量
     int u = T; int flow = INF;
     while(u != S) {
         Edge &e = edge[pre[u]];
         if(e.cap < flow) flow = e.cap;
         u = e.u;
     } u = T;
     while(u != S) {
         Edge& e1 = edge[pre[u]];
         Edge& e2 = edge[pre[u]^];
         e1.cap -= flow; e2.cap += flow;
         u = e1.u;
     }
     return flow;
 }

 int ISAP(int n) {
     if(!BFS()) return ; // 从汇点到源点逆向BFS初始化dis数组
     memset(num, , sizeof(num));
     memset(cur, , sizeof(cur)); // 当前弧优化
     for(int i = ; i <= n; i++)
         if(~dis[i]) num[dis[i]]++; // 当前距离为dis[i]的结点数
     int ans = , u = S;
     while(dis[S] < n) {
         if(u == T) ans += DFS(), u = S; //  回溯之前的结点并把更新流量
         int flag = ;
         for(int i = ; i < G[u].size(); i++) {
             Edge &e = edge[G[u][i]];
             if(dis[e.v] +  == dis[u] && e.cap > ) { // 可以增广
                 pre[e.v] = G[u][i]; cur[u] = i;
                 flag = ; u = e.v;
                 break;
             }
         }
         if(!flag) { // 不能增广,retreat
             if(--num[dis[u]] == ) break; // gap优化
             int md = n;
             for(int i = ; i < G[u].size(); i++) {
                 Edge &e = edge[G[u][i]];
                 if(e.cap >  && dis[e.v] < md) {
                     md = dis[e.v]; cur[u] = i;
                 }
             }
             num[dis[u] = md + ]++;
             if(u != S) u = edge[pre[u]].u;
         }
     }
     return ans;
 }

 int main() {
     int t;
     scanf("%d", &t);
     while(t--) {
         int n, m;
         scanf("%d%d", &n, &m);
         tot = ; for(int i = ; i <= n; i++) G[i].clear();
         int u, v, c, west = , east = -;
         for(int i = ; i <= n; i++) {
             scanf("%d%d", &u, &v);
             if(west > u) west = u, S = i;
             if(east < u) east = u, T = i;
         }
         for(int i = ; i <= m; i++) {
             scanf("%d%d%d", &u, &v, &c);
             Add(u, v, c); // 无向图的话反向边的cap也是c
         }
         printf("%d\n", ISAP(n));
     }
     return ;
 }