這里的最長公共子串是指的連續的子串，并非經典dp的那種。

aabbc

abc

這兩個字符串的最長公共子串為ab

算法復雜度是o(n)的（n為兩個字符串長度的和）

#include<iostream>
#include<cmath>
#include<cstring>
using namespace std;
const int maxn=210000;
char s[maxn];
int len,k;
int sa[maxn],rank[maxn],h[maxn],height[maxn];
int num[maxn];
inline bool leq(int a1, int a2, int b1, int b2)
{
	return (a1 < b1 || a1 == b1 && a2 <= b2);
}

inline bool leq(int a1, int a2, int a3, int b1, int b2, int b3)
{
	return(a1 < b1 || a1 == b1 && leq(a2, a3, b2, b3));
}

static void radixPass(int* a, int* b, int* r, int n, int K)
{
	int* c = new int[K + 1];
	for(int i = 0; i <= K; i++) c[i] = 0;
	for(int i = 0; i < n; i++) c[r[a[i]]]++;
	for(int i = 0, sum = 0; i <= K; i++)
	{
		int t = c[i]; c[i] = sum; sum += t;
	}
	for(int i = 0; i < n; i++) b[c[r[a[i]]]++] = a[i];
	delete [] c;
}

void suffixArray(int* T, int* SA, int n, int K)
{
	int n0 = (n + 2) / 3, n1 = (n + 1) / 3, n2 = n / 3, n02 = n0 + n2;
	int* R = new int[n02 + 3]; R[n02] = R[n02+1] = R[n02 + 2] = 0;
	int* SA12 = new int[n02 + 3]; SA12[n02] = SA12[n02 + 1] = SA12[n02 + 2] = 0;
	int* R0 = new int[n0];
	int* SA0 = new int[n0];
	for(int i = 0, j = 0; i < n + (n0 - n1); i++) if(i % 3 != 0) R[j++] = i;
	radixPass(R , SA12, T + 2, n02, K);
	radixPass(SA12, R , T + 1, n02, K);
	radixPass(R , SA12, T , n02, K);
	int name = 0, c0 = -1, c1 = -1, c2 = -1;
	for(int i = 0; i < n02; i++)
	{
		if(T[SA12[i]] != c0 || T[SA12[i] + 1] != c1 || T[SA12[i] + 2] != c2)
		{
			name++; c0 = T[SA12[i]]; c1 = T[SA12[i] + 1]; c2 = T[SA12[i] + 2];
		}
		if(SA12[i] % 3 == 1) { R[SA12[i] / 3] = name; }
		else{ R[SA12[i] / 3 + n0] = name; }
	}
	if(name < n02)
	{
		suffixArray(R, SA12, n02, name);
		for(int i = 0; i < n02; i++) R[SA12[i]] = i + 1;
	}
	else
		for(int i = 0; i < n02; i++) SA12[R[i] - 1] = i;
	for(int i = 0, j = 0; i < n02; i++) if(SA12[i] < n0) R0[j++] = 3 * SA12[i];
	radixPass(R0, SA0, T, n0, K);
	for(int p = 0, t = n0 - n1, k = 0; k < n; k++)
	{
		#define GetI() (SA12[t] < n0 ? SA12[t] * 3 + 1 : (SA12[t] - n0) * 3 + 2)
		int i = GetI();
		int j = SA0[p];
		if(SA12[t] < n0 ?
		leq(T[i], R[SA12[t] + n0], T[j], R[j / 3]) : leq(T[i],T[i + 1],R[SA12[t] - n0 + 1], T[j],T[j + 1],R[j / 3 + n0]))
		{
			SA[k] = i; t++;
			if(t == n02)
				for(k++; p < n0; p++, k++) SA[k] = SA0[p];
		}
		else{
			SA[k] = j;
			if(++p == n0)for(k++; t < n02; t++, k++) SA[k] = GetI();
		}
	}
	delete [] R; delete [] SA12; delete [] SA0; delete [] R0;
}

int main(){

 int i,n,a,b,j;

 scanf("%s",s+1);
 n=strlen(s+1);
 s[++n]='A'-1;
 scanf("%s",s+n+1);
 len=strlen(s+1);
 for(i=1;i<=len;i++)num[i]=s[i]-'A'+1;
 suffixArray(&num[1], &sa[1], len, 60);
 for(i=1;i<=len;i++)rank[++sa[i]]=i;
  for(i=1;i<=len;i++){
  a=i;b=sa[rank[i]-1];
  if(rank[i]==1)h[i]=0;
  else
  if(i==1||h[i-1]<=1){
   for(j=0;;j++)if(s[a+j]!=s[b+j])break;
   h[i]=j;
  }
  else{
   for(j=h[i-1]-1;;j++)if(s[a+j]!=s[b+j])break;
   h[i]=j;
  }
 }
  for(i=1;i<=len;i++)height[i]=h[sa[i]];
  int ans=0;
   for(i=2;i<=len;i++){
 	if(sa[i]<n&&sa[i-1]>n||sa[i]>n&&sa[i-1]<n)
 		if(height[i]>ans)ans=height[i];
 }
  printf("%d\n",ans);

 return 0;
}