[TJOI2015]弦论 后缀自动机

字典序第 k 大

 

沿着自动机的边走即可，比较水的一道题吧.

 

Code:

#include <cstdio>
#include <algorithm> 
#include <cstring>
#define maxn 1000000 
#define N 30
#define setIO(s) freopen(s".in","r",stdin)
using namespace std;
char str[maxn]; 
int opt,kth,n; 
struct SAM{
    int tot,last;
    int ch[maxn][N],f[maxn],cnt[maxn],mx[maxn],C[maxn],rk[maxn],sumv[maxn]; 
    void init(){tot=last=1;}
    void ins(int c) {
        int np = ++tot,p = last; last = np; mx[np] = mx[p] + 1;
        while(p && !ch[p][c]) ch[p][c] = np,p = f[p];
        if(!p) f[np] = 1;
        else {
            int q = ch[p][c];
            if(mx[q] == mx[p] + 1) f[np] = q;
            else {
                int nq = ++tot;
                f[nq] = f[q]; f[q] = f[np] = nq; 
                mx[nq] = mx[p] + 1;
                memcpy(ch[nq], ch[q], sizeof(ch[q])); 
                while(p && ch[p][c] == q) ch[p][c] = nq, p = f[p]; 
            }
        }
        cnt[np] = 1; 
    }
    void Solve() {
        for(int i = 1;i <= tot; ++i) ++C[mx[i]];
        for(int i = 1;i <= tot; ++i) C[i] += C[i - 1]; 
        for(int i = 1;i <= tot; ++i) rk[C[mx[i]]--] = i; 
        for(int i = tot;i >= 1; --i) { 
            int p = rk[i];
            if(opt == 1) cnt[f[p]] += cnt[p], sumv[p] = cnt[p]; 
            else sumv[p] = cnt[p] = 1; 
            for(int i = 0;i < 29; ++i) sumv[p] += sumv[ch[p][i]];  
        }
    }
    char ans[maxn]; 
    int id; 
    void BFS(int k) {
        cnt[1] = sumv[1] = 0; 
        int p = 1,con = 1,flag2 = 1;  
        while(con && flag2) {
            int flag = 1;
            flag2 = 0; 
            for(int i = 0;i < 29; ++i) {
                if(flag == 1) {
                    flag = 0; 
                    if(k <= cnt[p]) {con = 0; break;}
                    else k -= cnt[p]; 
                }
                //printf("%d %d\n",k,p);
                if(k > sumv[ch[p][i]]) k -= sumv[ch[p][i]];
                else { ans[++id] = i + 'a',flag2 = 1, p = ch[p][i]; break; }
            }        
        }
        if(!con) {
            for(int i = 1;i <= id; ++i) printf("%c",ans[i]); 
        }else printf("-1"); 
    }
}T;
int main(){
    //setIO("input");
    T.init(),scanf("%s",str),n = strlen(str); 
    for(int i = 0;i < n; ++i) T.ins(str[i] - 'a'); 
    scanf("%d",&opt),T.Solve(),scanf("%d",&kth),T.BFS(kth);  
    return 0; 
}