FOJ 题目 2075 Substring （后缀数组求出现k次的最小字典序子串）

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" alt="" />、

<pre name="code" class="cpp">#include<stdio.h>           
#include<string.h>           
#include<algorithm>           
#include<iostream>          
#define min(a,b) (a>b?b:a)       
#define max(a,b) (a>b?a:b)        
using namespace std;          
char str[103030];        
int sa[103030],Rank[103030],rank2[103030],height[103030],c[103030],*x,*y;      
int n;      
void cmp(int n,int sz)      
{      
    int i;      
    memset(c,0,sizeof(c));      
    for(i=0;i<n;i++)      
        c[x[y[i]]]++;      
    for(i=1;i<sz;i++)      
        c[i]+=c[i-1];      
    for(i=n-1;i>=0;i--)      
        sa[--c[x[y[i]]]]=y[i];      
}      
void build_sa(char *s,int n,int sz)      
{      
    x=Rank,y=rank2;      
    int i,j;      
    for(i=0;i<n;i++)      
        x[i]=s[i],y[i]=i;      
    cmp(n,sz);      
    int len;      
    for(len=1;len<n;len<<=1)      
    {      
        int yid=0;      
        for(i=n-len;i<n;i++)      
        {      
            y[yid++]=i;      
        }      
        for(i=0;i<n;i++)      
            if(sa[i]>=len)      
                y[yid++]=sa[i]-len;      
            cmp(n,sz);      
        swap(x,y);      
        x[sa[0]]=yid=0;      
        for(i=1;i<n;i++)      
        {      
            if(y[sa[i-1]]==y[sa[i]]&&sa[i-1]+len<n&&sa[i]+len<n&&y[sa[i-1]+len]==y[sa[i]+len])      
                x[sa[i]]=yid;      
            else      
                x[sa[i]]=++yid;      
        }      
        sz=yid+1;      
        if(sz>=n)      
            break;      
    }      
    for(i=0;i<n;i++)      
        Rank[i]=x[i];      
}      
void getHeight(char *s,int n)      
{      
    int k=0;      
    for(int i=0;i<n;i++)      
    {      
        if(Rank[i]==0)      
            continue;      
        k=max(0,k-1);      
        int j=sa[Rank[i]-1];      
        while(s[i+k]==s[j+k])      
            k++;      
        height[Rank[i]]=k;      
    }      
}    
int minv[103010][20],lg[103030];      
void init_lg()    
{    
    int i;    
    lg[1]=0;    
    for(i=2;i<102020;i++)    
    {    
        lg[i]=lg[i>>1]+1;    
    }    
}    
void init_RMQ(int n)    
{    
    int i,j,k;    
    for(i=1;i<=n;i++)    
    {    
        minv[i][0]=height[i];    
    }    
    for(j=1;j<=lg[n];j++)      
    {      
        for(k=0;k+(1<<j)-1<=n;k++)      
        {      
            minv[k][j]=min(minv[k][j-1],minv[k+(1<<(j-1))][j-1]);       
        }      
    }    
}    
int lcp(int l,int r)    
{    
    l=Rank[l];    
    r=Rank[r];    
    if(l>r)    
        swap(l,r);    
    l++;    
    int k=lg[r-l+1];    
    return min(minv[l][k],minv[r-(1<<k)+1][k]);      
}   
void solve(int n,int k)
{
	int i,j;
	for(i=1;i<=n;i++)
	{
		if(i+k-1>n)
			break;
		int t=lcp(sa[i],sa[i+k-1]);
		if(t>height[i]&&(i+k>n||t+k<=n&&t>height[i+k]))
		{
			for(j=0;j<t;j++)
			{
				printf("%c",str[j+sa[i]]);
			}
			printf("\n");
			return;
		}
	}
	printf("impossible\n");
}
int main()
{
	int n;
	while(scanf("%d",&n)!=EOF)
	{
		scanf("%s",str);
		int len=strlen(str);
		build_sa(str,len+1,128);
		getHeight(str,len);
		init_lg();
		init_RMQ(len);
		solve(len,n);
	}
}