bzoj 2561: 最小生成树
最小割，，，用比L小的搞到最小生成树，然最小割一下，那么在加上L后就会使图联通变为树，就可以了。最大同理，答案加起来
1 #include<bits/stdc++.h> 2 #define N 200005 3 #define LL long long 4 #define inf 0x3f3f3f3f 5 #define ls tr[x][0] 6 #define rs tr[x][1] 7 using namespace std; 8 inline int ra() 9 { 10 int x=0,f=1; char ch=getchar(); 11 while (ch<'0' || ch>'9') {if (ch=='-') f=-1; ch=getchar();} 12 while (ch>='0' && ch<='9') {x=x*10+ch-'0'; ch=getchar();} 13 return x*f; 14 } 15 struct node{ 16 int to,next,v; 17 }e[N<<3]; 18 int head[N],cnt=1; 19 struct data{ 20 int x,y,v; 21 }a[N]; 22 int h[N],q[N<<3],U,V,L,ans,n,m; 23 void in(int x, int y, int v) 24 { 25 e[++cnt].to=y; e[cnt].v=v; e[cnt].next=head[x]; head[x]=cnt; 26 } 27 void insert(int x, int y, int v) 28 { 29 in(x,y,v); in(y,x,1); //双向边2333 30 } 31 bool bfs() 32 { 33 for (int i=1; i<=n; i++) h[i]=-1; 34 h[U]=0; q[0]=U; int l=0,r=1; 35 while (l<r) 36 { 37 int x=q[l++]; 38 for (int i=head[x];i;i=e[i].next) 39 if (h[e[i].to]==-1 && e[i].v) 40 { 41 h[e[i].to]=h[x]+1; 42 q[r++]=e[i].to; 43 } 44 } 45 if (h[V]==-1) return 0; 46 return 1; 47 } 48 int dfs(int x, int f) 49 { 50 if (x==V) return f; 51 int used=0,w; 52 for (int i=head[x];i;i=e[i].next) 53 if (e[i].v && h[e[i].to]==h[x]+1) 54 { 55 w=dfs(e[i].to,min(f-used,e[i].v)); 56 used+=w; e[i].v-=w; e[i^1].v+=w; 57 if (used==f) return f; 58 } 59 if (!used) h[x]=-1; 60 return used; 61 } 62 void dinic() {while (bfs()) ans+=dfs(U,inf);} 63 bool cmp(data a, data b) 64 { 65 return a.v<b.v; 66 } 67 int main() 68 { 69 n=ra(); m=ra(); 70 for (int i=1; i<=m; i++) 71 a[i].x=ra(),a[i].y=ra(),a[i].v=ra(); 72 sort(a+1,a+m+1,cmp); 73 U=ra(),V=ra(),L=ra(); 74 for (int i=1; i<=m; i++) 75 if (a[i].v<L) insert(a[i].x,a[i].y,1); 76 else break; 77 dinic(); cnt=1; 78 memset(head,0,sizeof(head)); 79 for (int i=m; i>=1; i--) 80 if (a[i].v>L) insert(a[i].x,a[i].y,1); 81 else break; 82 dinic(); cout<<ans; 83 return 0; 84 }
