poj 2104 K-th Number (划分树入门)

题意：给n个数,m次询问，每次询问L到R中第k小的数是哪个

算法:划分树

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<iostream>
 5 using namespace std;
 6
 7 const int mx=1e5+5;
 8 int tree[30][mx];
 9 int sortt[mx];
10 int sum[30][mx];
11
12 void build(int l,int r,int c)
13 {
14     if (l==r) return ;
15     int m=(l+r)>>1;
16     int isame=m-l+1;
17     int pl=l,pr=m+1;
18     for (int i=l;i<m;i++)
19         if (tree[c][i]<sortt[m]) isame--;
20     for (int i=l;i<=r;i++){
21         if (i==l) sum[c][i]=0;
22         else sum[c][i]=sum[c][i-1];
23         if (tree[c][i]<sortt[m]){
24             tree[c+1][pl++]=tree[c][i];
25             sum[c][i]++;
26         }
27         else if ( tree[c][i]==sortt[m]&&isame){
28             isame--;
29             tree[c+1][pl++]=tree[c][i];
30             sum[c][i]++;
31         }
32         else tree[c+1][pr++]=tree[c][i];
33     }
34     build(l,m,c+1);
35     build(m+1,r,c+1);
36 }
37
38 int query(int L,int R,int l,int r,int c,int k){
39     if (L==R) return tree[c][L];
40     int s,ss;
41     if (l==L){
42         s=0;
43         ss=sum[c][r];
44     }
45     else{
46         s=sum[c][l-1];
47         ss=sum[c][r]-s;
48     }
49     int m=(L+R)>>1;
50     if (k<=ss) return query(L,m,L+s,L+s+ss-1,c+1,k);
51     return query(m+1,R,m+1+l-L-s,m+1+r-L-s-ss,c+1,k-ss);
52 }
53
54 int main(){
55     int n,m;
56     scanf("%d%d",&n,&m);
57     for (int i=1;i<=n;i++){
58         scanf("%d",&sortt[i]);
59         tree[0][i]=sortt[i];
60     }
61     sort(sortt+1,sortt+1+n);
62     build(1,n,0);
63     while (m--)
64     {
65         int l,r,k;
66         scanf("%d%d%d",&l,&r,&k);
67         printf("%d\n",query(1,n,l,r,0,k));
68     }
69
70 }