POJ 2104 K-th Number(划分树)

题目链接

参考HH大神的模版。对其中一些转移，还没想清楚，大体明白上是怎么回事了，划分树就是类似快排，但有点点区别的。多做几个题，慢慢理解。

 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 using namespace std;
 #define lson l,m,rt<<1
 #define rson m+1,r,rt<<1|1
 #define N 100100
 struct node
 {
     int l,r;
 }tree[*N];
 int sorted[N];
 int lnum[][N];
 int val[][N];

 void build(int d,int l,int r,int rt)
 {
     int m,lsame,i,lpos,rpos,same;
     tree[rt].l = l;
     tree[rt].r = r;
     if(l == r) return ;
     m = (l+r)>>;
     lsame = (m - l + );
     for(i = l;i <= r;i ++)
     {
         if(val[d][i] < sorted[m])
         lsame --;
     }
     lpos = l;
     rpos = m+;
     same = ;
     for(i = l;i <= r;i ++)
     {
         if(i == l)//lnum[d][i]表示[tree[rt].l, i ]区域里有多少个数分到左边
         lnum[d][i] = ;
         else
         lnum[d][i] = lnum[d][i-];
         if(val[d][i] < sorted[m])
         {
             lnum[d][i] ++;
             val[d+][lpos++] = val[d][i];
         }
         else if(val[d][i] > sorted[m])
         {
             val[d+][rpos++] = val[d][i];
         }
         else
         {
             if(same < lsame)
             {
                 same ++;
                 lnum[d][i] ++;
                 val[d+][lpos++] = val[d][i];
             }
             else
             {
                 val[d+][rpos++] = val[d][i];
             }
         }
     }
     build(d+,lson);
     build(d+,rson);
 }
 int query(int d,int k,int l,int r,int rt)
 {
     int s,ss,nl,nr,m,rs,rss;//s表示[ l , r ]有多少个分到左边
     if(l == r)//ss表示 [tree[idx].left , l-1 ]有多少个分到左边
     {
         return val[d][l];
     }
     if(l == tree[rt].l)
     {
         s = lnum[d][r];
         ss = ;
     }
     else
     {
         s = lnum[d][r] - lnum[d][l-];
         ss = lnum[d][l-];
     }
     if(s >= k)
     {
         nl = ss + tree[rt].l;
         nr = ss + tree[rt].l + s - ;
         return query(d+,k,nl,nr,rt<<);
     }
     else
     {
         m = (tree[rt].l + tree[rt].r)>>;
         rss = l - tree[rt].l - ss;
         rs = r - l +  - s;
         nl = m + rss + ;
         nr = m + rs + rss;
         return query(d+,k-s,nl,nr,rt<<|);
     }
 }
 int main()
 {
     int i,n,m,x,y,k;
     while(scanf("%d%d",&n,&m)!=EOF)
     {
         for(i = ;i <= n;i ++)
         {
             scanf("%d",&val[][i]);
             sorted[i] = val[][i];
         }
         sort(sorted+,sorted+n+);
         build(,,n,);
         for(i = ;i < m;i ++)
         {
             scanf("%d%d%d",&x,&y,&k);
             printf("%d\n",query(,k,x,y,));
         }
     }
     return ;
 }