BZOJ 1602 Usaco 牧场行走

LCA的模板题目，今天才发现自己其实对LCA这个算法一点都没领悟。之前一直是在套模板，所以今天的主要目标就是学习一下tarjan算法求LCA，顺便刷点LCA的相关习题来加强理解~

 1 #include <cstdio>
 2 #include <algorithm>
 3  
 4 int dis[1005],head[2333],fa[1100][21],deep[1005];
 5 int Count,n,q;
 6  
 7  
 8 struct node{
 9     int v,next,w;
10     node(){}
11     node(int _v,int _next,int _w){
12         v = _v;
13         next = _next;
14         w = _w;
15     }
16 }Edge[2333];
17  
18 void AddEdge(int u,int v,int w){
19     Count++;
20     Edge[Count] = node(v,head[u],w);
21     head[u] = Count;
22 }
23  
24 void dfs(int now,int f){
25     for(int i=head[now];i;i=Edge[i].next){
26         if(f==Edge[i].v) continue;
27         deep[Edge[i].v] = deep[now]+1;
28         fa[Edge[i].v][0]=now;
29         dis[Edge[i].v]=dis[now]+Edge[i].w;
30         dfs(Edge[i].v,now);
31     }
32 }
33  
34 int lca(int x,int y){
35     if(deep[x]<deep[y]) std::swap(x,y);
36     int k = deep[x] - deep[y];
37     for(int i=20;i>=0&&k;i--){
38         if((1<<i)&k){
39             x = fa[x][i];
40             k-=(1<<i);
41         }
42     }
43     if(x==y) return x;
44     for(int i=20;i>=0;i--){
45         if(fa[x][i]!=fa[y][i]){
46             x = fa[x][i];
47             y = fa[y][i];
48         }
49     }
50     if(x!=y) x = fa[x][0];
51     return x;
52 }
53  
54 int main(){
55     //freopen("fuckyou.out","w",stdout);
56     scanf("%d%d",&n,&q);
57     for(int i=1;i<n;i++){
58         int x,y,w;
59         scanf("%d%d%d",&x,&y,&w);
60         AddEdge(x,y,w);
61         AddEdge(y,x,w);
62     }
63     dfs(1,-1);
64     for(int j=1;j<=20;j++)
65         for(int i=1;i<=n;i++)
66             fa[i][j] = fa[fa[i][j-1]][j-1];
67     while(q--){
68         int x,y;
69         scanf("%d%d",&x,&y);
70         printf("%d\n",dis[x]+dis[y]-2*dis[lca(x,y)]);
71     }
72     return 0;
73 }