Lexicographical Substring Search (spoj7259) (sam)
Little Daniel loves to play with strings! He always finds different ways to have fun with strings! Knowing that, his friend Kinan decided to test his skills so he gave him a string \(S\) and asked him \(Q\) questions of the form:
If all distinct substrings of string \(S\) were sorted lexicographically, which one will be the \(K-th\)
After knowing the huge number of questions Kinan will ask, Daniel figured out that he can't do this alone. Daniel, of course, knows your exceptional programming skills, so he asked you to write him a program which given \(S\) will answer Kinan's questions.
Example:
\(S\)= "aaa" (without quotes)
substrings of S are "a" , "a" , "a" , "aa" , "aa" , "aaa". The sorted list of substrings will be:
"a", "aa", "aaa".
Input
In the first line there is Kinan's string \(S\) (with length no more than 90000 characters). It contains only small letters of English alphabet. The second line contains a single integer \(Q\) (\(Q<=500\)) , the number of questions Daniel will be asked. In the next \(Q\) lines a single integer \(K\)
Output
Output consists of \(Q\) lines, the i-th contains a string which is the answer to the i-th asked question.
Example
Input:
aaa
2
2
3
Output:
aa
aaa
Edited: Some input file contains garbage at the end. Do not process them.
題意:
給定一個字串,求排名第k小的串。
題解:
把串塞進一個字尾自動機,在圖上反向拓撲求出每個點的後繼串的數量,然後像在權值線段樹上找第\(k\)大一樣找就行了。
#include<bits/stdc++.h>
using namespace std;
const int N=2000010;
char s[N];
int a[N],c[N],ans[N];
struct SAM{
int last,cnt;
int size[N],ch[N][26],fa[N<<1],l[N<<1];
void ins(int c){
int p=last,np=++cnt;last=np;l[np]=l[p]+1;
for(;p&&!ch[p][c];p=fa[p])ch[p][c]=np;
if(!p)fa[np]=1;
else{
int q=ch[p][c];
if(l[p]+1==l[q])fa[np]=q;
else{
int nq=++cnt;l[nq]=l[p]+1;
memcpy(ch[nq],ch[q],sizeof ch[q]);
fa[nq]=fa[q];fa[q]=fa[np]=nq;
for(;ch[p][c]==q;p=fa[p])ch[p][c]=nq;
size[nq]=1;
}
}
size[np]=1;
}
void build(char s[]){
int len=strlen(s+1);
last=cnt=1;
for(int i=1;i<=len;++i)ins(s[i]-'a');
}
void calc(int len){
for(int i=1;i<=cnt;++i)c[l[i]]++;
for(int i=1;i<=cnt;++i)c[i]+=c[i-1];
for(int i=1;i<=cnt;++i)a[c[l[i]]--]=i;
for(int i=cnt;i;--i){
int p=a[i];
for(int j=0;j<26;++j){
size[p]+=size[ch[p][j]];
}
}
}
void find(int k){
int p=1;
while(k){
int a=0;
while(k>size[ch[p][a]]&&a<26){
if (ch[p][a]) k-=size[ch[p][a]];
a++;
}
putchar('a'+a);k--;
p=ch[p][a];
}
}
}sam;
int main(){
cin>>s+1;
sam.build(s);
sam.calc(strlen(s+1));
int t;
cin>>t;
while(t--){
int x;
scanf("%d",&x);
sam.find(x);putchar('\n');
}
}