[bzoj1787][Ahoi2008]Meet 紧急集合（lca）

传送门

可以看出，三个点两两之间的lca会有一对相同，而另一个lca就是聚集点。

然后搞搞就可以求出距离了。

——代码

 #include <cstdio>
 #include <cstring>
 #include <iostream>
 #define MAXN 1000001

 using namespace std;

 int n, m, cnt, ans;
 int head[MAXN], to[MAXN], next[MAXN], deep[MAXN], f[MAXN][];

 inline void add(int x, int y)
 {
     to[cnt] = y;
     next[cnt] = head[x];
     head[x] = cnt++;
 }

 inline void dfs(int u)
 {
     int i, v;
     deep[u] = deep[f[u][]] + ;
     for(i = ; f[u][i]; i++) f[u][i + ] = f[f[u][i]][i];
     for(i = head[u]; i != -; i = next[i])
     {
         v = to[i];
         if(!deep[v]) f[v][] = u, dfs(v);
     }
 }

 inline int lca(int x, int y)
 {
     int i;
     if(deep[x] < deep[y]) swap(x, y);
     for(i = ; i >= ; i--)
         if(deep[f[x][i]] >= deep[y])
             x = f[x][i];
     if(x == y) return x;
     for(i = ; i >= ; i--)
         if(f[x][i] != f[y][i])
             x = f[x][i], y = f[y][i];
     return f[x][];
 }

 int main()
 {
     int i, x, y, z, a;
     scanf("%d %d", &n, &m);
     memset(head, -, sizeof(head));
     for(i = ; i < n; i++)
     {
         scanf("%d %d", &x, &y);
         add(x, y);
         add(y, x);
     }
     dfs();
     for(i = ; i <= m; i++)
     {
         scanf("%d %d %d", &x, &y, &z);
         a = lca(x, y) ^ lca(x, z) ^ lca(y, z);
         ans = deep[x] + deep[a] -  * deep[lca(x, a)];
         ans += deep[y] + deep[a] -  * deep[lca(y, a)];
         ans += deep[z] + deep[a] -  * deep[lca(z, a)];
         printf("%d %d\n", a, ans);
     }
     return ;
 }