[CodeVS2370] 小机房的树 (LCA)

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 struct edge
 4 {
 5     int v, w, nxt;
 6 }e[100005];
 7 int fst[50005], q[50005], front, back;
 8 int fa[17][50005], dis[50005], dep[50005];
 9 
10 void addedge(int i, int u, int v, int w)
11 {
12     e[i] = (edge){v, w, fst[u]}, fst[u] = i;
13 }
14 
15 int LCA(int u, int v)
16 {
17     if(dep[u] > dep[v]) swap(u, v);
18     for(int i = 16; ~i; i--)
19         if(dep[fa[i][v]] >= dep[u])
20             v = fa[i][v];
21     if(u == v) return u;
22     for(int i = 16; ~i; i--)
23         if(fa[i][u] != fa[i][v])
24             u = fa[i][u], v = fa[i][v];
25     return fa[0][u];
26 }
27 
28 int main()
29 {
30     int n, m, u, v, w;
31     cin >> n;
32     for(int i = 1; i < n; i++)
33     {
34         cin >> u >> v >> w;
35         addedge(i << 1, ++u, ++v, w);
36         addedge(i << 1 | 1, v, u, w);
37     }
38     q[++back] = 1, dep[1] = fa[0][1] = 1;
39     while(front != back)
40     {
41         u = q[++front];
42         for(int i = fst[u]; i; i = e[i].nxt)
43             if(e[i].v != fa[0][u])
44             {
45                 q[++back] = e[i].v;
46                 dep[e[i].v] = dep[u] + 1;
47                 fa[0][e[i].v] = u;
48                 dis[e[i].v] = dis[u] + e[i].w;
49             }
50     }
51     for(int i = 1; i <= 16; i++)
52         for(int j = 1; j <= n; j++)
53             fa[i][j] = fa[i - 1][fa[i - 1][j]];
54     cin >> m;
55     while(m--)
56     {
57         cin >> u >> v, u++, v++;
58         cout << dis[u] + dis[v] - 2 * dis[LCA(u, v)] << endl;
59     }
60     return 0;
61 }