[luoguP2912] [USACO08OCT]牧场散步Pasture Walking（lca）

传送门

水题。

直接倍增求lca。

x到y的距离为dis[x] + dis[y] - 2 * dis[lca(x, y)]

——代码

 #include <cstdio>
 #include <cstring>
 #include <iostream>
 #define MAXN 20002

 using namespace std;

 int n, q, cnt;
 int head[MAXN], to[MAXN], next[MAXN], val[MAXN], deep[MAXN], f[MAXN][], dis[MAXN];

 inline void add(int x, int y, int z)
 {
     to[cnt] = y;
     val[cnt] = z;
     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;
             dis[v] = dis[u] + val[i];
             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;
     scanf("%d %d", &n, &q);
     memset(head, -, sizeof(head));
     for(i = ; i < n; i++)
     {
         scanf("%d %d %d", &x, &y, &z);
         add(x, y, z);
         add(y, x, z);
     }
     deep[] = ;
     dfs();
     for(i = ; i <= q; i++)
     {
         scanf("%d %d", &x, &y);
         printf("%d\n", dis[x] + dis[y] -  * dis[lca(x, y)]);
     }
     return ;
 }