http://codevs.cn/problem/1993/ (题目链接)
题意
求有向图最大流。
Solution
Dinic。
代码
// codevs1993
#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<cstdio>
#include<cmath>
#include<queue>
#define LL long long
#define inf 2147483640
#define MOD 10000
#define Pi acos(-1.0)
#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
using namespace std; const int maxn=210;
struct edge {int to,next,w;}e[maxn<<1];
int d[maxn],head[maxn],cnt=1,ans,n,m; void link(int u,int v,int w) {
e[++cnt]=(edge){v,head[u],w};head[u]=cnt;
e[++cnt]=(edge){u,head[v],0};head[v]=cnt;
}
bool bfs() {
memset(d,-1,sizeof(d));
queue<int> q;q.push(1);d[1]=0;
while (!q.empty()) {
int x=q.front();q.pop();
for (int i=head[x];i;i=e[i].next) if (e[i].w && d[e[i].to]<0) {
d[e[i].to]=d[x]+1;
q.push(e[i].to);
}
}
return d[n]>0;
}
int dfs(int x,int f) {
if (x==n || f==0) return f;
int w,used=0;
for (int i=head[x];i;i=e[i].next) if (e[i].w && d[e[i].to]==d[x]+1) {
w=dfs(e[i].to,min(e[i].w,f-used));
used+=w;
e[i].w-=w;e[i^1].w+=w;
if (used==f) return used;
}
if (!used) d[x]=-1;
return used;
}
void Dinic() {
while (bfs()) ans+=dfs(1,inf);
}
int main() {
scanf("%d%d",&m,&n);
for (int u,v,w,i=1;i<=m;i++) {
scanf("%d%d%d",&u,&v,&w);
link(u,v,w);
}
Dinic();
printf("%d",ans);
return 0;
}