poj1273&hdu1523Drainage Ditches(ISAP)

题目请戳这里

题目大意：略。

题目分析：网络流模版题。不过数据很弱，只能测很烂的模版。

第一道网络流

详情请见代码：

#include <iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int N = 205;
const int M = 410;
const int inf = 0x3f3f3f3f;

struct node
{
    int to,next,pre,c,f;
}arc[M];
int num;
int head[N];
int que[N];//bfs用
int sta[N];//保存当前弧
int rpath[N];//保存反向弧
int cnt[N];
int dis[N];
int m,n;
void build(int s,int e,int cap)//建图
{
    arc[num].to = e;
    arc[num].c = cap;
    arc[num].f = 0;
    arc[num].next = head[s];
    head[s] = num ++;
    arc[num - 1].pre = num;//反向弧
    arc[num].pre = num - 1;
    arc[num].to = s;
    arc[num].c = 0;
    arc[num].f = 0;
    arc[num].next = head[e];
    head[e] = num ++;
}
void re_Bfs()
{
    int i,front,rear;
    front = rear = 0;
    for(i = 1;i <= n;i ++)
    {
        dis[i] = inf;
        cnt[i] = 0;
    }
    que[rear ++] = n;
    cnt[0] = 1;
    dis[n] = 0;
    while(front != rear)
    {
        int u = que[front ++];
        for(i = head[u];i != -1;i = arc[i].next)
        {
            if(arc[arc[i].pre].c == 0 || dis[arc[i].to] < inf)
                continue;
            dis[arc[i].to] = dis[u] + 1;
            cnt[dis[arc[i].to]] ++;
            que[rear ++] = arc[i].to;
        }
    }
}
int ISAP()
{
    int i,u,v,ret = 0;
    u = 1;
    for(i = 1;i <= n;i ++)
        sta[i] = head[i];
    while(dis[1] < n)
    {
        if(u == n)
        {
            int curflow = inf;
            for(i = 1;i != n;i = arc[sta[i]].to)
                curflow = min(curflow,arc[sta[i]].c);
            for(i = 1;i != n;i = arc[sta[i]].to)
            {
                arc[sta[i]].c -= curflow;
                arc[arc[sta[i]].pre].c += curflow;
                arc[sta[i]].f += curflow;
                arc[arc[sta[i]].pre].f -= curflow;
            }
            ret += curflow;
            u = 1;
        }
        for(i = sta[u];i != -1;i = arc[i].next)//寻找允许弧
            if(arc[i].c > 0 && dis[arc[i].to] + 1 == dis[u])
                break;
        if(i != -1)
        {
            sta[u] = i;
            rpath[arc[i].to] = arc[i].pre;
            u = arc[i].to;
        }
        else
        {
            if((--cnt[dis[u]]) == 0)//gap优化
                break;
            sta[u] = head[u];
            int Min = N;
            for(i = head[u];i != -1;i = arc[i].next)
                if(arc[i].c > 0)
                    Min = min(Min,dis[arc[i].to]);
            dis[u] = Min + 1;
            cnt[dis[u]] ++;
            if(u != 1)
                u = arc[rpath[u]].to;
        }
    }
    return ret;
}
int main()
{
    int i,u,v,c;
    while(scanf("%d%d",&m,&n) != EOF)
    {
        memset(head,-1,sizeof(head));
        num = 0;
        while(m --)
        {
            scanf("%d%d%d",&u,&v,&c);
            build(u,v,c);
        }
        re_Bfs();
        printf("%d\n",ISAP());
    }
    return 0;
}