【模板】lca 最近公共祖先

lca：

#include<cstring>#include<algorithm>using namespace std;const int MAXN=500001;int n,m,gen,x,y;struct Edge{    int next,to;}edge[2*MAXN];int deep[MAXN],fa[MAXN][20];//deep记录每个点的深度，fa[i][j]:表示节点向上跳2^j 个节点所能到达的节点//显然fa[i][0]就是直接的父节点了 int num_edge,head[MAXN];void add_edge(int from,int to){    edge[++num_edge].next=head[from];    edge[num_edge].to=to;    head[from]=num_edge;}bool vis[MAXN];void dfs(int x)//保存f[i][0] {    vis[x]=true;    for (int i=1; i<=16; i++)    {        if ((1<<i)>deep[x]) break;        fa[x][i]=fa[fa[x][i-1]][i-1];    }    for (int i=head[x]; i!=0; i=edge[i].next)    {        if (!vis[edge[i].to])        {            deep[edge[i].to]=deep[x]+1;            fa[edge[i].to][0]=x;            dfs(edge[i].to);        }    }}int lca(int x,int y){    if (deep[x]<deep[y]) swap(x,y);    int d=deep[x]-deep[y];    for (int i=0; i<=16; i++)    {        if ((1<<i)&d) x=fa[x][i];//判断d的第i位是否为1，是的话就往上跳     }    if (x==y) return x;    for (int i=16; i>=0; i--)//因为要找最深的公共祖先，所以要倒着循环     {        if (fa[x][i]!=fa[y][i])        {            x=fa[x][i];//一起往上跳到最近公共祖先的下面             y=fa[y][i];        }    }    return fa[x][0];}int main(){    scanf("%d%d%d",&n,&m,&gen);    for (int i=1; i<=n-1; i++)    {        scanf("%d%d",&x,&y);        add_edge(x,y); add_edge(y,x);    }    dfs(gen);    for (int i=1; i<=m; i++)    {        scanf("%d%d",&x,&y);        printf("%d\n",lca(x,y));    }    return 0;}/*5 5 43 12 45 11 42 43 23 51 24 544144*/