[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

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <iostream>
 4 #define MAXN 20002
 5
 6 using namespace std;
 7
 8 int n, q, cnt;
 9 int head[MAXN], to[MAXN], next[MAXN], val[MAXN], deep[MAXN], f[MAXN][21], dis[MAXN];
10
11 inline void add(int x, int y, int z)
12 {
13     to[cnt] = y;
14     val[cnt] = z;
15     next[cnt] = head[x];
16     head[x] = cnt++;
17 }
18
19 inline void dfs(int u)
20 {
21     int i, v;
22     deep[u] = deep[f[u][0]] + 1;
23     for(i = 0; f[u][i]; i++) f[u][i + 1] = f[f[u][i]][i];
24     for(i = head[u]; i != -1; i = next[i])
25     {
26         v = to[i];
27         if(!deep[v])
28         {
29             f[v][0] = u;
30             dis[v] = dis[u] + val[i];
31             dfs(v);
32         }
33     }
34 }
35
36 inline int lca(int x, int y)
37 {
38     int i;
39     if(deep[x] < deep[y]) swap(x, y);
40     for(i = 20; i >= 0; i--)
41         if(deep[f[x][i]] >= deep[y])
42             x = f[x][i];
43     if(x == y) return x;
44     for(i = 20; i >= 0; i--)
45         if(f[x][i] != f[y][i])
46             x = f[x][i], y = f[y][i];
47     return f[x][0];
48 }
49
50 int main()
51 {
52     int i, x, y, z;
53     scanf("%d %d", &n, &q);
54     memset(head, -1, sizeof(head));
55     for(i = 1; i < n; i++)
56     {
57         scanf("%d %d %d", &x, &y, &z);
58         add(x, y, z);
59         add(y, x, z);
60     }
61     deep[1] = 1;
62     dfs(1);
63     for(i = 1; i <= q; i++)
64     {
65         scanf("%d %d", &x, &y);
66         printf("%d\n", dis[x] + dis[y] - 2 * dis[lca(x, y)]);
67     }
68     return 0;
69 }