问题描述
LG1344 https://www.luogu.org/problem/P1344
题解
我太菜了, 我一开始竟然没有看出这是个最小割裸题...
两个询问.
第一个询问, 直接跑最小割就好了.
第二个询问, 建图的时候边权建 \(1\) , 代表割掉这条边需要 \(1\) 的代价.
- \(\mathrm{Code}\)
- #include<bits/stdc++.h>
- using namespace std;
- template <typename Tp>
- void read(Tp &x){
- x=0;char ch=1;int fh;
- while(ch!='-'&&(ch>'9'||ch<'0')) ch=getchar();
- if(ch=='-') ch=getchar(),fh=-1;
- else fh=1;
- while(ch>='0'&&ch<='9') x=(x<<1)+(x<<3)+ch-'0',ch=getchar();
- x*=fh;
- }
- const int maxn=37;
- const int maxm=2007;
- int n,m,S,T;
- struct Graph{
- int Head[maxn],to[maxm],Next[maxm],w[maxm],tot=1;
- Graph(){
- memset(Head,0,sizeof(Head));
- memset(Next,0,sizeof(Next));
- }
- void add(int x,int y,int z){
- to[++tot]=y,Next[tot]=Head[x],Head[x]=tot,w[tot]=z;
- }
- }G[2];
- int d[maxn];
- bool bfs(int gp){
- memset(d,0,sizeof(d));
- queue<int>q;q.push(S);d[S]=1;
- while(!q.empty()){
- int x=q.front();q.pop();
- for(int i=G[gp].Head[x];i;i=G[gp].Next[i]){
- int y=G[gp].to[i];
- if(d[y]||!G[gp].w[i]) continue;
- q.push(y);d[y]=d[x]+1;
- if(y==T) return true;
- }
- }
- return false;
- }
- int dfs(int x,int flow,int gp){
- if(x==T) return flow;
- int REST=flow;
- for(int i=G[gp].Head[x];i&&REST;i=G[gp].Next[i]){
- int y=G[gp].to[i];
- if(d[y]!=d[x]+1||!G[gp].w[i]) continue;
- int k=dfs(y,min(REST,G[gp].w[i]),gp);
- if(!k) d[y]=0;
- else{
- G[gp].w[i]-=k,G[gp].w[i xor 1]+=k;
- REST-=k;
- }
- }
- return flow-REST;
- }
- int ans;
- int main(){
- read(n);read(m);
- for(int i=1,x,y,z;i<=m;i++){
- read(x);read(y);read(z);
- G[1].add(x,y,z);G[1].add(y,x,0);
- G[0].add(x,y,1);G[0].add(y,x,0);
- }
- S=1,T=n;
- while(bfs(1)){
- int t;
- while(t=dfs(S,0x3f3f3f3f,1)) ans+=t;
- }
- printf("%d",ans);ans=0;
- while(bfs(0)){
- int t;
- while(t=dfs(S,0x3f3f3f3f,0)) ans+=t;
- }
- printf("%d\n",ans);
- return 0;
- }
来源: http://www.bubuko.com/infodetail-3268520.html