codevs 2370 小机房的树(LCA)

 

过了这么长的时间终于开始看LCA了...

有一次训练题卡在LCA当时不会...拖了好久好久...其实现在还是不会...

只会tarjan...

传送门

板子题咯

tarjan的算法就是基于先序遍历的顺序的

#include <bits/stdc++.h>
using namespace std;

inline int read() {
    int x = 0, f = 1; char ch = getchar();
    while (ch < '0' || ch > '9') { if (ch == '-') f = -1; ch = getchar(); }
    while (ch >= '0' && ch <= '9') { x = x * 10 + ch - '0'; ch = getchar(); }
    return x * f;
}

const int maxn = 5e5 + 10;
//const int maxm = 8e4 + 10;

struct Edge { int to, next, c; } edge[2 * maxn];
int cnt, head[maxn];
struct Qedge { int to, next; } qedge[2 * maxn];
int qcnt, qhead[maxn], n, m;
int lca[maxn], dep[maxn], par[maxn];
bool vis[maxn];
inline void addedge(int u, int v, int c) {
    edge[++cnt].to = v;
    edge[cnt].c = c;
    edge[cnt].next = head[u];
    head[u] = cnt; 
}
inline void addqedge(int u, int v) {
    qedge[++qcnt].to = v;
    qedge[qcnt].next = qhead[u];
    qhead[u] = qcnt;
}
int getfa(int x) { return x == par[x] ? x : par[x] = getfa(par[x]); }

void dfs(int u) {
    par[u] = u;
    vis[u] = 1; 
    for (int i = head[u]; i; i = edge[i].next) {
        int v = edge[i].to;
        if (!vis[v]) {
            dep[v] = dep[u] + edge[i].c;
            dfs(v);
            par[v] = u;
        } 
    }
    for (int i = qhead[u]; i; i = qedge[i].next) {
        int v = qedge[i].to;
        if (vis[v]) {
            lca[i] = getfa(v);
            if (i % 2) lca[i + 1] = lca[i];
            else lca[i-1] = lca[i];
        }
    }
}

int main() {
    n = read();
    for (int i = 0; i < n - 1; i++) {
        int u = read(), v =read(), c = read();
        addedge(u, v, c);
        addedge(v, u, c);
    }
    m = read();
    for (int i = 0; i < m; i++) {
        int u = read(), v = read();
        addqedge(u, v);
        addqedge(v, u);
    }
    dfs(0);
    for (int i = 1; i <= m; i++) {
        int ans = dep[qedge[2 * i].to] - 2 * dep[lca[2 * i]] + dep[qedge[2 * i - 1].to];
        printf("%d\n", ans); 
    } 
    return 0;
}