不会 准备研究一波！！！
1 #include<bits/stdc++.h>
2 const int maxn = 500010;
3 using namespace std;
4 vector<int> g[maxn];
5 int par[20][maxn], dep[maxn], n, m, ml;
6 void dfs(int v, int p, int d)
7 {
8 par[0][v] = p;
9 dep[v] = d;
10 for (int i = 0; i < g[v].size(); ++i){
11 if (g[v][i] != p) dfs(g[v][i], v, d + 1);
12 }
13 return ;
14 }
15 void init(int v)
16 {
17 dfs(v, -1, 0);
18 int sum = 1;
19 while (sum <= n) sum <<= 1, ++ml;
20 for (int k = 0; k < ml; ++k){
21 for (int v = 1; v <= n; ++v){
22 if (par[k][v] == -1) par[k + 1][v] = -1;
23 else par[k + 1][v] = par[k][par[k][v]];
24 }
25 }
26 return ;
27 }
28 int lca(int u, int v)
29 {
30 if (dep[u] > dep[v]) swap(u, v);
31 for (int k = 0; k < ml; ++k){
32 if (((dep[v] - dep[u]) >> k) & 1) v = par[k][v];
33 }
34 if (u == v) return u;
35 for (int k = ml; k >= 0; --k){
36 if (par[k][u] != par[k][v]) u = par[k][u], v = par[k][v];
37 }
38 return par[0][u];
39 }
40 int main()
41 {
42 int s, u, v;
43 scanf("%d %d %d", &n, &m, &s);
44 int t = n;
45 while (--t){
46 scanf("%d %d", &u, &v);
47 g[u].push_back(v);
48 g[v].push_back(u);
49 }
50 init(s);
51 while (m--){
52 scanf("%d %d", &u, &v);
53 printf("%d\n", lca(u, v));
54 }
55 return 0;
56 }
题目 https://www.luogu.org/problemnew/show/P3379
自己写的一个倍增LCA
1 #include<iostream>
2 #include<cstring>
3 //#include<bits/stdc++.h>
4 #include<math.h>
5 #include<algorithm>
6 #include<queue>
7 #include<stack>
8 #include<cstdio>
9 #include<map>
10 #include<set>
11 #define si(a) scanf("%d",&a)
12 #define sl(a) scanf("%lld",&a)
13 #define sii(a,b) scanf("%d%d",&a,&b)
14 #define sll(a,b) scanf("%lld%lld",&a,&b)
15 #define queues priority_queue
16 #define mod 998244353
17 #define mem(a) memset(a,0,sizeof(a));
18 #define def(a) ((a)&(-a))
19 #define fi first
20 #define se second
21 #define mp make_pair
22 #define pb push_back
23 typedef long long ll;
24 //priority_queue<int,vector<int >,greater<int > >q;
25 const ll INF=0x3f3f3f3f;
26 //const double E=exp(1);
27 //const double PI=acos(-1);
28 using namespace std;
29 int de[500003];
30 int lg[500003];
31 int f[500003][30];
32 vector<int>ss[500003];
33 void get_lg()
34 {lg[0]=-1;
35 for(int i=1;i<=500000;i++)
36 lg[i]=lg[i>>1]+1;
37 }
38 void dfs(int p,int fa)
39 { de[p]=de[fa]+1;
40 f[p][0]=fa;
41 for(int i=1;i<=lg[de[p]]+1;i++)
42 f[p][i]=f[f[p][i-1]][i-1];
43 for(int i=0;i<ss[p].size();i++)
44 { int x=ss[p][i];
45 if(x!=fa)
46 {
47 dfs(x,p);
48 }
49 }
50 }
51 int LCA(int a,int b)
52 { if(de[a]<de[b])swap(a,b);
53 while(de[a]!=de[b])
54 {
55 a=f[a][lg[de[a]-de[b]]];
56 }
57 if(a==b)return a;
58 for(int i=lg[de[a]];i>=0;i--)
59 {
60 if(f[a][i]!=f[b][i])
61 a=f[a][i],b=f[b][i];
62 }
63 return f[a][0];
64 }
65 int main()
66 { ios::sync_with_stdio(false);
67 int n,m,s;
68 scanf("%d%d%d",&n,&m,&s);
69 for(int i=1;i<n;i++)
70 {
71 int a,b;
72 scanf("%d%d",&a,&b);
73 ss[a].pb(b);
74 ss[b].pb(a);
75 }
76 get_lg();
77 dfs(s,0);
78 for(int i=1;i<=n;i++)
79 {
80 int a,b;
81 scanf("%d%d",&a,&b);
82 printf("%d\n",LCA(a,b));
83 // cout<<LCA(a,b)<<endl;
84 }
85 }
又有一个树链剖分求LCA
1 #include<bits/stdc++.h>
2 typedef long long ll;
3 const int maxn=500000+5;
4 const int INF=0x3f3f3f3f;
5 using namespace std;
6 const ll MAX=100000
7 int read(){int x=0,f=1;char s=getchar();for(; s>'9'||s<'0'; s=getchar()) if(s=='-') f=-1;for(; s>='0'&&s<='9'; s=getchar()) x=x*10+s-'0';return x*f;}
8 vector<int>q[maxn];
9 int f[maxn];
10 int d[maxn];
11 int siz[maxn];
12 int son[maxn];
13
14 int top[maxn];
15 int id[maxn];
16 int rk[maxn];
17 int dfs(int a,int fa)
18 {
19 f[a]=fa;
20 d[a]=d[fa]+1;
21 siz[a]=1;
22 for(int i=0; i<q[a].size(); i++)
23 {
24 int z=q[a][i];
25 if(z!=fa)
26 {
27 dfs(z,a);
28 siz[a]+=siz[z];
29 if(!son[a]||siz[z]>siz[son[a]])
30 son[a]=z;
31 }
32 }
33 return 0;
34 }
35 int sum;
36 void dfs1(int a,int b)
37 {
38 top[a]=b;
39 if(!son[a])
40 return ;
41 dfs1(son[a],b);
42 for(int i=0; i<q[a].size(); i++)
43 {
44 int z=q[a][i];
45 if(z!=f[a]&&z!=son[a])
46 dfs1(z,z);
47 }
48 }
49 int lca(int a,int b)
50 {
51 while(top[a]!=top[b])
52 {
53 d[top[a]]>d[top[b]]?a=f[top[a]]:b=f[top[b]];
54
55 }
56 return d[a]<d[b]?a:b;
57 }
58 int main()
59 {
60 ios::sync_with_stdio(false);
61 int n,m,s;
62 n=read();m=read();s=read();
63 for(int i=1; i<n; i++)
64 {
65 int a,b;
66 a=read();
67 b=read();
68 q[a].pb(b);
69 q[b].pb(a);
70 }
71 dfs(s,0);
72 dfs1(s,s);
73 while(m--)
74 {
75 int a,b;
76 a=read();
77 b=read();
78 cout<<lca(a,b)<<endl;
79 }
80 }
来源：https://www.cnblogs.com/zxz666/p/10669536.html