lca 倍增模版

Posted 2021-02-04 dwvictor

 
const int POW = 18;
void dfs(int u,int fa){
d[u]=d[fa]+1;
    p[u][0]=fa;
for(int i=1;i<POW;i++) p[u][i]=p[p[u][i-1]][i-1];
int sz=edge[u].size();
for(int i=0;i<sz;i++){
int v=edge[u][i];
if(v==fa) continue;
dfs(v,u);
}
}
int lca( int a, int b ){
if( d[a] > d[b] ) a ^= b, b ^= a, a ^= b;
if( d[a] < d[b] ){
int del = d[b] - d[a];
for( int i = 0; i < POW; i++ ) if(del&(1<<i)) b=p[b][i];
}
if( a != b ){
for( int i = POW-1; i >= 0; i-- )
if( p[a][i] != p[b][i] )
     a = p[a][i] , b = p[b][i];
a = p[a][0], b = p[b][0];
}
return a;
}