P3379 【模板】最近公共祖先（LCA）

题目传送门：https://www.luogu.com.cn/problem/P3379

倍增LCA模板：

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<algorithm>
 4 #include<vector>
 5 #include<map>
 6 #include<queue>
 7 #include<set>
 8 #include<cmath>
 9 #include<list>
10 #include<cstring>
11 #include<string>
12 #define ll long long
13 #define ull unsigned long long
14 #define inf 0x3f3f3f3f
15 #define inff 0x7fffffff
16 using namespace std;
17 const int N = 500000 + 10;
18 
19 int fa[N][31], dep[N];
20 vector<int>G[N];
21 
22 void dfs(int x, int pre) {
23 
24     fa[x][0] = pre;
25     dep[x] = dep[pre] + 1;
26     for (int i = 1; i <= 30; i++) {
27         fa[x][i] = fa[fa[x][i - 1]][i - 1];
28     }
29     for (int i = 0; i < G[x].size(); i++) {
30         if (G[x][i] != pre) dfs(G[x][i], x);
31     }
32 
33 }
34 
35 int lca(int x, int y) {
36 
37     if (dep[x] < dep[y]) swap(x, y);
38     for (int i = 30; i >= 0; i--) {
39         if ((1 << i) <= dep[x] - dep[y]) x = fa[x][i];
40     }
41     if (x == y) return x;
42     for (int i = 30; i >= 0; i--) {
43         if (fa[x][i] != fa[y][i]) {
44             x = fa[x][i];
45             y = fa[y][i];
46         }
47     }
48 
49     return fa[x][0];
50 }
51 
52 int main() {
53 
54     int n, m, s;
55     cin >> n >> m >> s;
56     for (int i = 1; i < n; i++) {
57         int u, v;
58         cin >> u >> v;
59         G[u].push_back(v);
60         G[v].push_back(u);
61     }
62     dfs(s, 0);
63     for (; m; --m) {
64         int x, y;
65         cin >> x >> y;
66         cout << lca(x, y) << "\n";
67     }
68 
69     return 0;
70 }