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;}