视频链接:
P3629 [APIO2010] 巡逻 - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)
// 两次DFS+树形DP O(n) #include <iostream> #include <cstring> #include <algorithm> using namespace std;const int N=100005; int idx=1,head[N],to[N<<1],ne[N<<1],w[N<<1]; void add(int x,int y){to[++idx]=y,w[idx]=1,ne[idx]=head[x],head[x]=idx; } int d[N],fa[N],col[N]; int n,k,p,st,ed,d1,d2;void dfs(int x,int f){fa[x]=f; //记录路径if(d[x]>d[p]) p=x; //更新端点for(int i=head[x];i;i=ne[i]){int y=to[i];if(y==f)continue;d[y]=d[x]+w[i];dfs(y,x);} } void change(int x,int f){for(int i=head[x];i;i=ne[i]){int y=to[i];if(y==f)continue;if(col[y])w[i]=-1,w[i^1]=-1;change(y,x);} } void dfs2(int x,int f){for(int i=head[x];i;i=ne[i]){int y=to[i];if(y==f)continue;dfs2(y,x);d2=max(d2,d[y]+d[x]+w[i]);d[x]=max(d[x],d[y]+w[i]);} } int main(){scanf("%d%d",&n,&k);for(int i=1,x,y;i<n;++i){scanf("%d%d",&x,&y);add(x,y),add(y,x);}dfs(1,0); st=p; d[p]=0;dfs(p,0); ed=p; d1=d[p]; //两次DFSfor(int i=ed;i;i=fa[i])col[i]=1; //直径染色change(st,0); //修改直径的边权memset(d,0,sizeof d);dfs2(st,0); //树形DPif(k==1) printf("%d\n",2*n-d1-1);else printf("%d\n",2*n-d1-d2); }