[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][21], 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][0]] + 1;
    for(i = 0; f[u][i]; i++) f[u][i + 1] = f[f[u][i]][i];
    for(i = head[u]; i != -1; i = next[i])
    {
        v = to[i];
        if(!deep[v])
        {
            f[v][0] = 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 = 20; i >= 0; i--)
        if(deep[f[x][i]] >= deep[y])
            x = f[x][i];
    if(x == y) return x;
    for(i = 20; i >= 0; i--)
        if(f[x][i] != f[y][i])
            x = f[x][i], y = f[y][i];
    return f[x][0];
}

int main()
{
    int i, x, y, z;
    scanf("%d %d", &n, &q);
    memset(head, -1, sizeof(head));
    for(i = 1; i < n; i++)
    {
        scanf("%d %d %d", &x, &y, &z);
        add(x, y, z);
        add(y, x, z);
    }
    deep[1] = 1;
    dfs(1);
    for(i = 1; i <= q; i++)
    {
        scanf("%d %d", &x, &y);
        printf("%d
", dis[x] + dis[y] - 2 * dis[lca(x, y)]);
    }
    return 0;
}