HDU 4007 Dave 线段树

题意：广场上有许多的人，现在给你一个边长为R的正方形，求正方形最多能框住的人，包括边界。
题解：

#include <algorithm>  #include <iostream>  using namespace std;    #define N 2010  #define L(u) ( u * 2 )  #define R(u) ( u * 2 + 1 )  #define max(a,b) ( a > b ? a : b )    unsigned int Y[N];    struct TreeNode  {      int l, r, v, add;  } node[N*3];    struct Point  {      unsigned int x, y;int v;  } point[N];    bool cmp ( Point & a, Point & b )  {      if ( a.x == b.x )          return a.v > b.v;     return a.x < b.x;  }    int bfind ( int l, int r, unsigned int key )  {      while ( l <= r )      {          int mid = ( l + r ) / 2;          if ( Y[mid] == key )              return mid;          if ( key < Y[mid] )              r = mid - 1;          else              l = mid + 1;      }      return -1;  }    void build ( int u, int l, int r )  {      node[u].l = l;      node[u].r = r;      node[u].v = node[u].add = 0;      if ( l == r ) return;      int mid = ( l + r ) / 2;      build ( L(u), l, mid );      build ( R(u), mid + 1, r );  }    void update ( int u, int l, int r, int val )  {      if ( l <= node[u].l && node[u].r <= r )      {          node[u].add += val;          node[u].v += val;          return;      }        if ( node[u].add != 0 )      {          node[L(u)].add += node[u].add;          node[L(u)].v += node[u].add;          node[R(u)].add += node[u].add;          node[R(u)].v += node[u].add;          node[u].add = 0;      }                int mid = ( node[u].l + node[u].r ) / 2;        if ( r <= mid )           update ( L(u), l, r, val );      else if ( l > mid )          update ( R(u), l, r, val );      else      {          update ( L(u), l, mid, val );          update ( R(u), mid + 1, r, val );      }      node[u].v = max ( node[L(u)].v, node[R(u)].v );  }    int main()  {      int n, R, x, y, i, j;      while ( scanf("%d%d",&n,&R) != EOF )      {          for ( i = 1; i <= n; i++ )          {              scanf("%d%d",&x,&y);              point[i].x = x;              point[i].y = y;              point[i].v = 1;              Y[i] = y;          }             for ( i = 1; i <= n; i++ )          {              point[i+n].x = point[i].x + R;              point[i+n].y = point[i].y;              point[i+n].v = -1;              Y[i+n] = point[i].y + R;          }            sort ( point+1, point+1+n*2, cmp );          sort ( Y+1, Y+1+n*2 );                    for ( i = j = 1; i <= n*2; i++ )          {              if ( Y[j] != Y[i] )                  Y[++j] = Y[i];          }            build ( 1, 1, j );               int l, r, ans = 0;            for ( i = 1; i <= n * 2; i++ )          {              l = bfind ( 1, j, point[i].y );              r = bfind ( 1, j, point[i].y + R );              update ( 1, l, r, point[i].v );              if ( ans < node[1].v ) ans = node[1].v;          }          printf("%d\n", ans );      }      return 0;  }