国庆七天乐_day2 bzoj3998弦论(后缀自动机)

传送门

 

题意：求字典序第k小的字串

对于T为0的情况，每个状态我们计数都为1

对于T为1的情况，对于每个状态他的计数应加上他fail树结束节点的个数(实际的对应串的个数)

然后随便DFS就好了

 

#pragma GCC optimize(2)
#pragma GCC optimize(3)
#pragma GCC optimize(4)
#include
using namespace std;
typedef long long ll;

const int N = 5e5 + 10;
struct SAM
{
    static const int KN = N << 1;
    static const int KM = 30;
    int fail[KN], net[KN][KM], len[KN], cnt, root;
    int val[KN],sum[KN];
    int newnode(int _len)
    {
        memset(net[cnt], -1, sizeof(net[cnt]));
        //fail[cnt] = -1;
        len[cnt] = _len;
        return cnt++;
    }
    void init()
    {
        cnt = 0;
        memset(fail,-1,sizeof(fail));
        root = newnode(0);
    }
    int add(int p, int x)
    {
        int np = newnode(len[p] + 1);
        val[np]=1;
        while(~p && net[p][x] == -1)    net[p][x] = np, p = fail[p];
        if(p == -1) fail[np] = root;
        else
        {
            int q = net[p][x];
            if(len[q] == len[p] + 1)    fail[np] = q;
            else
            {
                int nq = newnode(len[p] + 1);
                memcpy(net[nq], net[q], sizeof(net[q]));
                fail[nq] = fail[q];
                fail[q] = fail[np] = nq;
                while(~p && net[p][x] == q) net[p][x] = nq, p = fail[p];
            }
        }
        return np;
    }
    void build(char *s, char ch)
    {
        int now = root;
        for(int i = 0; s[i]; ++i)   now = add(now, s[i] - ch);
    }
    int ord[KN], pri[KN];
    void topo()
    {
        int maxVal=0;
        memset(pri, 0, sizeof(pri));
        for (int i = 0; i < cnt; ++i) maxVal = max(maxVal, len[i]), ++ pri[len[i]];
        for (int i = 1; i <= maxVal; ++i) pri[i] += pri[i - 1];
        for (int i = 0; i < cnt; ++i) ord[--pri[len[i]]] = i;
    }
    void sum_init(int t)
    {
        for(int i=cnt-1;i>=0;i--)
        {
            int pos=ord[i];
            if(t) val[fail[pos]]+=val[pos];
            else val[pos]=1;
        }
        val[0]=0;
        for(int i=cnt-1;i>=0;i--)
        {
            int pos=ord[i];
            sum[pos]=val[pos];
            for(int j=0;j<26;j++) sum[pos]+=sum[net[pos][j]];
        }
    }
    void dfs(int k,int x)
    {
        if(val[x]>=k) return ;
        else k-=val[x];
        for(int i=0;i<26;i++)
        {
            int pos=net[x][i];
            if(pos)
            {
                if(k<=sum[pos])
                {
                    printf("%c",i+'a');
                    dfs(k,pos);
                    return ;
                }
                else k-=sum[pos];
            }
        }
    }
    void gao(int t,int k)
    {
        topo();
        sum_init(t);
        dfs(k,0);
    }
} sam;

char s[N];
int main()
{
    scanf("%s",s);
    int t,k;
    scanf("%d%d",&t,&k);
    sam.init();
    sam.build(s,'a');
    sam.gao(t,k);
}