hdu 2586 How far away ？（LCA）

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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;}
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