hdu 2586 How far away ？（LCA）

题目链接：hdu 2586 How far away ？

代码

#include <cstdio>
#include <cstring>
#include <vector>
#include <queue>
#include <algorithm>

using namespace std;
typedef pair<int,int> pii;
const int maxn = 40005;
const int inf = 0x3f3f3f3f;
const int BIT = 20;

int dep[maxn], f[maxn][BIT+5];
vector<pii> G[maxn];

void ST (int n) {
    for (int k = 1; k <= BIT; k++) {
        for (int i = 1; i <= n; i++) 
            f[i][k] = f[f[i][k-1]][k-1];
    }

    int s;
    for (int i = 1; i <= n; i++) if (f[i][0] == 0) {
        s = i; break;
    }
    memset(dep, inf, sizeof(dep));
    dep[s] = 0;

    queue<int> que;
    que.push(s);
    while (!que.empty()) {
        int u = que.front();
        que.pop();

        for (int i = 0; i < G[u].size(); i++) {
            int v = G[u][i].first;
            if (dep[v] > dep[u] + 1) {
                dep[v] = dep[u] + 1;
                que.push(v);
            }
        }
    }
}

int LCA(int u, int v) {
    if (dep[u] > dep[v]) swap(u, v);

    int mv = dep[v] - dep[u];
    for (int i = 0; i <= BIT && mv; mv >>= 1, i++)
        if (mv&1) v = f[v][i];

    if (u == v) return u;

    for (int i = BIT; i >= 0; i--) {
        if (f[u][i] == f[v][i]) continue;
        u = f[u][i]; v = f[v][i];
    }
    return f[u][0];
}

int N, M, dis[maxn];

void dfs (int u, int fa) {
    f[u][0] = fa;

    for (int i = 0; i < G[u].size(); i++) {
        int v = G[u][i].first, w = G[u][i].second;
        if (v == fa) continue;
        dis[v] = dis[u] + w;
        dfs(v, u);
    }
}

void init () {
    scanf("%d%d", &N, &M);
    for (int i = 1; i <= N; i++) G[i].clear();
    memset(f, 0, sizeof(f));

    int u, v, w;
    for (int i = 1; i < N; i++) {
        scanf("%d%d%d", &u, &v, &w);
        G[u].push_back(make_pair(v, w));
        G[v].push_back(make_pair(u, w));
    }
}

int main () {
    int cas;
    scanf("%d", &cas);
    while (cas--) {
        init();
        dfs(1, 0);
        ST(N);

        int u, v;
        while (M--) {
            scanf("%d%d", &u, &v);
            int k = LCA(u, v);
            printf("%d\n", dis[u] - dis[k] + dis[v] - dis[k]);
        }
    }
    return 0;
}