【luogu P3379 最近公共祖先】 模板

题目链接：https://www.luogu.org/problemnew/show/P3379

倍增求lca，先存下板子，留个坑以后再填讲解。

in

5 5 4
3 1
2 4
5 1
1 4
2 4
3 2
3 5
1 2
4 5

out

4
4
1
4
4

 #include <cstdio>
 #include <iostream>
 #include <algorithm>
 #include <cstring>
 using namespace std;
 const int maxlog = ;
 const int maxn = ;
 int n, m, s;
 int root;
 int fa[maxn][maxlog];
 int deep[maxn];
 int head[maxn];
 int cnt;
 struct Edge{
     int next;
     int to;
 }e[ * maxn];
 void add(int u, int v)
 {
     e[cnt].to = v;
     e[cnt].next = head[u];
     head[u] = cnt++;
 }
 void dfs(int u, int p, int d)
 {
     fa[u][] = p;
     deep[u] = d;
     for(int i = head[u]; i != -; i = e[i].next)
         if(e[i].to != p) dfs(e[i].to, u, d+);
 }
 void init()
 {
     dfs (root, -, );
     for(int k = ; k +  < maxlog; k++)
     {
         for(int v = ; v <= n; v++)
         if(fa[v][k] < ) fa[v][k+] = -;
         else fa[v][k+] = fa[fa[v][k]][k];
     }
 }
 int lca(int u, int v)
 {
     if(deep[u] > deep[v]) swap(u, v);
     for(int k = ; k < maxlog; k++)
     {
         if((deep[v] - deep[u]) >> k & )
         v = fa[v][k];
     }
     if(u == v) return u;
     for(int k = maxlog - ; k >= ; k--)
     {
         if(fa[v][k] != fa[u][k])
         {
             u = fa[u][k];
             v = fa[v][k];
         }
     }
     return fa[u][];
 }
 int main()
 {
     memset(head,-,sizeof(head));
     int a,b;
     scanf("%d%d%d",&n,&m,&root);
     for(int i = ; i < n; i++)
     {
         scanf("%d%d",&a,&b);
         add(a,b);
         add(b,a);
     }
     init();
     for(int i = ; i <= m; i++)
     {
         int u,v,a;
         scanf("%d%d",&u,&v);
         a = lca(u,v);
         printf("%d\n",a);
     }
     return ;
 }
 /*
 in:
 9 5 1
 1 2
 1 3
 2 5
 2 4
 3 6
 7 9
 6 7
 6 8
 9 8
 9 5
 6 5
 4 3
 4 5
 out:
 6
 1
 1
 1
 2
 */