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 ", 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 ",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 }