uva10829 L-Gap Substrings

传送门：https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=20&page=show_problem&problem=1770

思路：这题和上一题SPOJ687的思路一样，也是枚举长度，再对于i和i+j+L向前向后匹配，但是这一题只能在后面接一个反串。

#include<cmath>
#include<cstdio>
#include<cstring>
#include<algorithm>
const int maxn=100010;
using namespace std;
int n,sum[maxn],t1[maxn],t2[maxn],sa[maxn],rank[maxn],h[maxn],st[maxn][20],L,len;char s[maxn];long long ans;

void getsa(){
	int *x=t1,*y=t2,m=255,p=0;
	for (int i=1;i<=n;i++) sum[x[i]=s[i]]++;
	for (int i=1;i<=m;i++) sum[i]+=sum[i-1];
	for (int i=n;i;i--) sa[sum[x[i]]--]=i;
	for (int j=1;p<n;j<<=1,m=p){
		p=0;
		for (int i=n-j+1;i<=n;i++) y[++p]=i;
		for (int i=1;i<=n;i++) if (sa[i]>j) y[++p]=sa[i]-j;
		memset(sum,0,sizeof(sum));
		for (int i=1;i<=n;i++) sum[x[y[i]]]++;
		for (int i=1;i<=m;i++) sum[i]+=sum[i-1];
		for (int i=n;i;i--) sa[sum[x[y[i]]]--]=y[i];
		swap(x,y),x[sa[1]]=p=1;
		for (int i=2;i<=n;i++){
			if (y[sa[i]]!=y[sa[i-1]]||y[sa[i]+j]!=y[sa[i-1]+j]) p++;
			x[sa[i]]=p;
		}
	}
	memcpy(rank,x,sizeof(rank));
}

void geth(){
	for (int i=1,j=0;i<=n;i++){
		if (rank[i]==1) continue;
		while (s[i+j]==s[sa[rank[i]-1]+j]) j++;
		h[rank[i]]=j;
		if (j) j--;
	}
}

void getst(){
	for (int i=1;i<=n;i++) st[i][0]=h[i];
	for (int j=1;j<=19;j++)
		for (int i=1;i+(1<<j)-1<=n;i++)
			st[i][j]=min(st[i][j-1],st[i+(1<<(j-1))][j-1]);
}

int getmin(int x,int y){
	int l=rank[x],r=rank[y];
	if (l>r) swap(l,r);
	l++;int k=log2(r-l+1);
	return min(st[l][k],st[r-(1<<k)+1][k]);
}

int main(){
	scanf("%d%s",&L,s+1);
	len=strlen(s+1),s[n=len+1]=127;
	for (int i=len;i;i--) s[++n]=s[i];s[n+1]=0;
	getsa(),geth(),getst();
	for (int i=1;i+i+L<=len;i++)
		for (int j=1;j<=len;j+=i){
			int k=j+i+L,tmp=0;
			if (k<=len) tmp+=min(i,getmin(j,k));
			if (j>=2) tmp+=min(i-1,getmin(n-j+2,n-k+2));
			ans+=max(0,tmp-i+1);
		}
	printf("%lld\n",ans);
	return 0;
}