2599: [IOI2011]Race
分析
被memset卡。。。
点分治，对于重心，遍历子树，记录一个数组T[i]，表示以重心为起点的长度为i的路径中最少的边数是多少。然后先遍历子树，更新答案，然后在遍历一边更新T，防止出现两个起点在同一棵子树中的情况。
代码
1 #include<cstdio> 2 #include<algorithm> 3 #include<cctype> 4 5 using namespace std; 6 7 const int N = 200100; 8 const int INF = 1e9; 9 10 struct Edge{ 11 int to,nxt,w; 12 Edge() {} 13 Edge(int a,int b,int c) {to = a,w = b,nxt = c;} 14 }e[N<<1]; 15 int head[N],siz[N],T[1000010]; 16 bool vis[N]; 17 int n,k,tot,Size,Num,Root,Ans = N; 18 19 inline int read() { 20 int x = 0,f = 1;char ch=getchar(); 21 for (; !isdigit(ch); ch=getchar()) if(ch=='-')f=-1; 22 for (; isdigit(ch); ch=getchar()) x=x*10+ch-'0'; 23 return x*f; 24 } 25 void add_edge(int u,int v,int w) { 26 e[++tot] = Edge(v,w,head[u]);head[u] = tot; 27 e[++tot] = Edge(u,w,head[v]);head[v] = tot; 28 } 29 void getRoot(int u,int fa) { 30 siz[u] = 1; 31 int mx = 0; 32 for (int i=head[u]; i; i=e[i].nxt) { 33 int v = e[i].to; 34 if (v == fa || vis[v]) continue; 35 getRoot(v,u); 36 siz[u] += siz[v]; 37 mx = max(siz[v],mx); 38 } 39 mx = max(Size-siz[u],mx); 40 if (mx < Num) Root = u,Num = mx; 41 } 42 void dfs1(int u,int fa,int L,int C) { 43 if (L > k) return ; 44 Ans = min(Ans,T[k-L]+C); 45 for (int i=head[u]; i; i=e[i].nxt) { 46 int v = e[i].to; 47 if (v == fa || vis[v]) continue; 48 dfs1(v,u,L+e[i].w,C+1); 49 } 50 } 51 void dfs2(int u,int fa,int L,int C,int f) { 52 if (L > k) return ; 53 if (f) T[L] = min(T[L],C); 54 else T[L] = INF; 55 for (int i=head[u]; i; i=e[i].nxt) { 56 int v = e[i].to; 57 if (v == fa || vis[v]) continue; 58 dfs2(v,u,L+e[i].w,C+1,f); 59 } 60 } 61 void calcc(int u) { 62 T[0] = 0; 63 for (int i=head[u]; i; i=e[i].nxt) { 64 int v = e[i].to; 65 if (vis[v]) continue; 66 dfs1(v,u,e[i].w,1); 67 dfs2(v,u,e[i].w,1,1); 68 } 69 for (int i=head[u]; i; i=e[i].nxt) { 70 int v = e[i].to; 71 if (vis[v]) continue; 72 dfs2(v,u,e[i].w,1,0); 73 } 74 } 75 void solve(int u) { 76 vis[u] = true; 77 calcc(u); 78 for (int i=head[u]; i; i=e[i].nxt) { 79 int v = e[i].to; 80 if (vis[v]) continue; 81 Size = siz[v],Root = 0,Num = 1e9; 82 getRoot(v,0); 83 solve(Root); 84 } 85 } 86 int main() { 87 n = read(),k = read(); 88 for (int i=1; i<=k; ++i) T[i] = INF; 89 for (int u,v,w,i=1; i<n; ++i) { 90 u = read(),v = read(),w = read(); 91 add_edge(u+1,v+1,w); 92 if (w == k) {printf("-1");return 0;} 93 } 94 Size = n;Num = 1e9; 95 getRoot(1,0); 96 solve(Root); 97 if (Ans == N) puts("-1"); 98 else printf("%d",Ans); 99 return 0; 100 }