Code[VS] 2370 LCA 题解

  1 /*
  2     LCA 
  3     author:SHHHS
  4     2016-09-26 02:25:19 
  5 */
  6 
  7 #include "bits/stdc++.h"
  8 
  9 using namespace std ;
 10 typedef long long QAQ ;
 11 struct Edge { int to , val , next ;};
 12 struct Tree { int l , r , mintr ,pos ;};
 13 int gmin ( int x , int y ) { return x > y ? y : x ;}
 14 int gmax ( int x , int y ) { return x < y ? y : x ;}
 15 const int maxN = 200010 ;
 16 const int INF = 2147483647 ;
 17 
 18 Tree tr[ maxN<<2 ] ;
 19 Edge e[ maxN ] ;
 20 int cnt , dfs_num=0 , min_val ,min_pos ;
 21 int head[maxN],fst[maxN],dep[maxN],ver[maxN],Dis[maxN];
 22 bool vis[maxN];
 23 
 24 inline void Add_Edge ( int x , int y , int _val ){
 25          e[ ++cnt ].to = y ;
 26          e[ cnt ].val = _val ;
 27          e[ cnt ].next = head[ x ] ;
 28          head[ x ] = cnt ;
 29 }
 30 
 31 void Build_Tree ( int x , int y , int i ) {
 32          tr[ i ].l = x ; tr[ i ].r = y ;
 33          if ( x==y ) tr[ i ].mintr = dep[ x ] , tr[ i ].pos = x ;
 34          else {
 35                   QAQ mid = ( tr[ i ].l + tr[ i ].r ) >>1 ;
 36                  Build_Tree ( x , mid , i<<1);
 37                  Build_Tree ( mid+1 , y , i<<1|1);
 38                  if (tr[i<<1].mintr > tr[i<<1|1].mintr )
 39                           tr[ i ].pos = tr[i<<1|1].pos,tr[ i ].mintr = tr[i<<1|1].mintr;
 40                  else 
 41                          tr[ i ].pos = tr[ i<<1 ].pos,tr[ i ].mintr = tr[ i<<1 ].mintr;
 42          }
 43          
 44 }
 45 
 46 void DFS ( int x , int depth ) {
 47          vis[ x ] = true ; 
 48          ver[ ++dfs_num ] = x ; 
 49          fst[ x ] = dfs_num ; 
 50          dep[ dfs_num ] = depth ;
 51          for ( int i=head[ x ] ; i ; i=e[i].next ) {
 52                   int temp = e[ i ].to ;
 53                   if ( !vis[ temp ] ){
 54                            Dis[ temp ] = Dis[ x ] + e[ i ].val ;
 55                            DFS ( temp , depth + 1 ) ;
 56                            ver[ ++dfs_num ] = x ; 
 57                            dep[ dfs_num ] = depth ;
 58                  }
 59          }
 60 }
 61 
 62 void Query_Min ( int q , int w , int i ) {
 63          if(q <= tr[i].l && w >= tr[i].r ){
 64                   if (tr[ i ].mintr < min_val ){
 65                            min_val = tr[i].mintr ;
 66                            min_pos = tr[i].pos ;
 67                   }
 68          }
 69          else {
 70                   QAQ mid = (tr[i].l + tr[i].r ) >> 1;
 71                   if(q > mid) {
 72                           Query_Min ( q , w , i << 1 | 1);
 73                   }
 74                   else if(w <= mid) {
 75                           Query_Min ( q , w , i << 1);
 76                   }
 77                   else {
 78                           Query_Min ( q , w , i << 1) ;
 79                           Query_Min ( q , w , i << 1 | 1);
 80                   }
 81          }
 82 }
 83 
 84 int LCA ( int x , int y ) {
 85          int px = fst[ x ] , py = fst[ y ] , tmp ;
 86          min_val = INF ;
 87          if ( py < px ) swap ( px , py ) ;
 88          Query_Min ( px , py , 1 ) ;
 89          return ver[ min_pos ] ;
 90 }
 91 int main ( ) {
 92          int N ,M ;
 93          scanf ("%d",&N);
 94          for ( int i=1 ; i<=N-1 ; ++i ) {
 95                   int _x , _y , __ ;
 96                   scanf("%d %d %d" , &_x , &_y ,&__ ) ;
 97                   ++_x;++_y;
 98                   Add_Edge ( _x , _y , __ ) ;
 99                   Add_Edge ( _y , _x , __ ) ;
100          }
101          DFS ( 1 , 1 ) ;
102          Build_Tree ( 1 , dfs_num , 1 ) ;
103          scanf ("%d",&M);
104          for ( int i=1 ; i<=M ; ++i ) {
105                   int u , v ;
106                   scanf ( "%d%d" , &u , &v ) ;
107                   ++u,++v;
108                   printf ("%d",Dis[u]+Dis[v]-2*Dis[LCA ( u , v )] ) ;
109                   putchar(‘\n‘);
110          }
111          return 0 ;
112 }