HDU3887 DFS序+ 线段树

时间:2024-07-31 22:36:20

查询树上某个节点的子节点的标号小于其标号的数目. 一个trick是建立线段树之后,从标号小的向标号大的来做更新.

   1:  #include <cstdio>
   2:  #include <cstring>
   3:  #include <cctype>
   4:  #include <algorithm>
   5:  #include <vector>
   6:  #include <iostream>
   7:  using namespace std;
   8:  #pragma comment(linker, "/STACK:1024000000,1024000000")
   9:  #define LL(a)   a<<1
  10:  #define RR(a)   a<<1|1
  11:   
  12:  const int MaxL = 200030;
  13:   
  14:  int pre[MaxL];  // first travel
  15:  int nxt[MaxL];  // nxt travel
  16:  int f[MaxL];
  17:  bool vis[MaxL];
  18:  int N;
  19:  vector<vector<int> > E(MaxL);
  20:   
  21:  struct Seg_tree
  22:  {
  23:      int left, right;
  24:      int sum;
  25:  } tt[MaxL<<2];
  26:   
  27:   
  28:  void PushUp(int idx)
  29:  {
  30:      tt[idx].sum = tt[LL(idx)].sum + tt[RR(idx)].sum;
  31:  }
  32:   
  33:  void build(int l,int r,int idx)
  34:  {
  35:      tt[idx].left = l, tt[idx].right = r;
  36:      tt[idx].sum = 0;
  37:      if (l == r) return ;
  38:      int m = (l + r) >> 1;
  39:      build(l,m, LL(idx));
  40:      build(m+1, r, RR(idx));
  41:  }
  42:   
  43:  void update(int p, int idx = 1)
  44:  {
  45:      if(p == tt[idx].left && p == tt[idx].right)
  46:      {
  47:          tt[idx].sum =1;
  48:          return ;
  49:      }
  50:      int mid = (tt[idx].left + tt[idx].right)>>1;
  51:      if(p <= mid) update(p, LL(idx));
  52:      else update(p, RR(idx));
  53:      PushUp(idx);
  54:  }
  55:   
  56:  int query(int l, int r, int idx = 1)
  57:  {
  58:      if(l == tt[idx].left && r ==tt[idx].right)
  59:          return tt[idx].sum;
  60:      int mid = (tt[idx].left + tt[idx].right)>>1;
  61:      if(r <= mid) return query(l,r, LL(idx));
  62:      else if(l> mid) return query(l,r, RR(idx));
  63:      else
  64:          return query(l, mid, LL(idx))+ query(mid+1, r, RR(idx));
  65:  }
  66:   
  67:  int mark = 1;
  68:  void dfs( int a)
  69:  {
  70:      vis[a] = 1;
  71:      pre[a] = mark++;
  72:      for(int i=0; i<E[a].size(); i++)
  73:      {
  74:          if(!vis[E[a][i]])
  75:              dfs(E[a][i]);
  76:      }
  77:      nxt[a] = mark++;
  78:  }
  79:   
  80:  int main()
  81:  {
  82:  //    freopen("1.txt","r",stdin);
  83:      int p;
  84:      while( scanf("%d%d", &N, &p) && N!=0 && p!=0)
  85:      {
  86:          memset(pre, 0, sizeof(pre));
  87:          memset(nxt, 0 ,sizeof(nxt));
  88:          memset(vis, 0 ,sizeof(vis));
  89:          for(int i=1; i<=N; i++) E[i].clear();
  90:   
  91:          for(int i=1; i<N; i++)
  92:          {
  93:              int a,b;
  94:              scanf("%d%d", &a,&b);
  95:              E[a].push_back(b);
  96:              E[b].push_back(a);
  97:          }
  98:          mark = 1;
  99:          dfs(p);
 100:          build(1, 2*N, 1);
 101:          for(int i=1; i<=N; i++)
 102:          {
 103:              f[i] =query(pre[i], nxt[i],1);
 104:              update(pre[i],1);
 105:          }
 106:          for(int i=1; i<N; i++)
 107:              printf("%d ",f[i]);
 108:          printf("%d\n",f[N]);
 109:      }
 110:      return 0;
 111:  }

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