# SPOJ: LCS – Longest Common Substring

Solution Idea:

This is the problem no 6 on Suffix array paper .
The question need to find the maximum value of LCP[] array between two consecutive suffixes where this two consecutive suffixes are from different input string.

```
#include &lt;bits/stdc++.h&gt;

#define pii              pair &lt;int,int&gt;
#define pll              pair &lt;long long,long long&gt;
#define sc               scanf
#define pf               printf
#define Pi               2*acos(0.0)
#define ms(a,b)          memset(a, b, sizeof(a))
#define pb(a)            push_back(a)
#define MP               make_pair
#define db               double
#define ll               long long
#define EPS              10E-10
#define ff               first
#define ss               second
#define sqr(x)           (x)*(x)
#define D(x)             cout&lt;&lt;#x &quot; = &quot;&lt;&lt;(x)&lt;&lt;endl
#define VI               vector &lt;int&gt;
#define DBG              pf(&quot;Hi\n&quot;)
#define MOD              1000000007
#define CIN              ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define SZ(a)            (int)a.size()
#define sf(a)            scanf(&quot;%d&quot;,&amp;a)
#define sfl(a)           scanf(&quot;%lld&quot;,&amp;a)
#define sff(a,b)         scanf(&quot;%d %d&quot;,&amp;a,&amp;b)
#define sffl(a,b)        scanf(&quot;%lld %lld&quot;,&amp;a,&amp;b)
#define sfff(a,b,c)      scanf(&quot;%d %d %d&quot;,&amp;a,&amp;b,&amp;c)
#define sfffl(a,b,c)     scanf(&quot;%lld %lld %lld&quot;,&amp;a,&amp;b,&amp;c)
#define stlloop(v)       for(__typeof(v.begin()) it=v.begin();it!=v.end();it++)
#define loop(i,n)        for(int i=0;i&lt;n;i++)
#define loop1(i,n)       for(int i=1;i&lt;=n;i++)
#define REP(i,a,b)       for(int i=a;i&lt;b;i++)
#define RREP(i,a,b)      for(int i=a;i&gt;=b;i--)
#define TEST_CASE(t)     for(int z=1;z&lt;=t;z++)
#define PRINT_CASE       printf(&quot;Case %d: &quot;,z)
#define LINE_PRINT_CASE  printf(&quot;Case %d:\n&quot;,z)
#define CASE_PRINT       cout&lt;&lt;&quot;Case &quot;&lt;&lt;z&lt;&lt;&quot;: &quot;
#define all(a)           a.begin(),a.end()
#define intlim           2147483648
#define infinity         (1&lt;&lt;28)
#define ull              unsigned long long
#define gcd(a, b)        __gcd(a, b)
#define lcm(a, b)        ((a)*((b)/gcd(a,b)))

using namespace std;

/*----------------------Graph Moves----------------*/
//const int fx[]={+1,-1,+0,+0};
//const int fy[]={+0,+0,+1,-1};
//const int fx[]={+0,+0,+1,-1,-1,+1,-1,+1};   // Kings Move
//const int fy[]={-1,+1,+0,+0,+1,+1,-1,-1};  // Kings Move
//const int fx[]={-2, -2, -1, -1,  1,  1,  2,  2};  // Knights Move
//const int fy[]={-1,  1, -2,  2, -2,  2, -1,  1}; // Knights Move
/*------------------------------------------------*/

//int Set(int N,int pos){return N=N | (1&lt;&lt;pos);}
//int reset(int N,int pos){return N= N &amp; ~(1&lt;&lt;pos);}
//bool check(int N,int pos){return (bool)(N &amp; (1&lt;&lt;pos));}
/*------------------------------------------------*/

#define mx 500005

int SA[mx],tempSA[mx];
int RA[mx],tempRA[mx];
int c[mx];
int Phi[mx],PLCP[mx],LCP[mx];
char str[mx],str1[mx];
int n;

void counting_sort(int k)
{
int maxi=max(n,300);
ms(c,0);
for(int i=0; i&lt;n; i++)
{
int a=i+k&lt;n?RA[i+k]:0;
c[a]++;
}
for(int i=0,sum=0; i&lt;maxi; i++)
{
int x=c[i];
c[i]=sum;
sum+=x;
}

for(int i=0; i&lt;n; i++)
{
int a=SA[i]+k&lt;n?RA[SA[i]+k]:0;
int b=c[a];
c[a]++;
tempSA[b]=SA[i];
}

for(int i=0; i&lt;n; i++)
SA[i]=tempSA[i];
}

void build_SA()
{
for(int i=0; i&lt;n; i++)
{
RA[i]=str[i];
SA[i]=i;
}

for(int k=1; k&lt;n; k*=2)
{
counting_sort(k);
counting_sort(0);

int r=0;
tempRA[SA[0]]=r=0;

for(int i=1; i&lt;n; i++)
{
if(RA[SA[i]]==RA[SA[i-1]] &amp;&amp; RA[SA[i]+k]==RA[SA[i-1]+k])
tempRA[SA[i]]=r;
else
tempRA[SA[i]]=++r;
}

for(int i=0; i&lt;n; i++)
RA[i]=tempRA[i];

if(RA[SA[n-1]]==n-1) break;
}
}

void build_LCP()
{
Phi[SA[0]]=-1;
for(int i=1; i&lt;n; i++)
Phi[SA[i]]=SA[i-1];
for(int i=0,L=0; i&lt;n; i++)
{
if(Phi[i]==-1)
{
PLCP[i]=0;
continue;
}
while(str[i+L]==str[Phi[i]+L]) L++;
PLCP[i]=L;
L=max(L-1,0);
}

for(int i=0; i&lt;n; i++)
LCP[i]=PLCP[SA[i]];

}

int main()
{

//    freopen(&quot;in.txt&quot;,&quot;r&quot;,stdin);
//	  freopen(&quot;out.txt&quot;,&quot;w&quot;,stdout);

int pos;

scanf(&quot;%s %s&quot;,str,str1);
strcat(str,&quot;\$&quot;);
pos=strlen(str)-1;
strcat(str,str1);
strcat(str,&quot;#&quot;);
n=strlen(str);
build_SA();
build_LCP();
//    string ss=str;
//    for(int i=0;i&lt;n;i++)
//        cout&lt;&lt;SA[i]&lt;&lt;&quot; &quot;;
//    cout&lt;&lt;endl;
//    for(int i=0;i&lt;n;i++)
//        cout&lt;&lt;LCP[i]&lt;&lt;&quot; &quot;;
//    cout&lt;&lt;endl;
//    for(int i=0;i&lt;n;i++)
//        cout&lt;&lt;ss.substr(SA[i])&lt;&lt;endl;

int ans=0;

for(int i=1; i&lt;n; i++)
{
if((SA[i]&lt;pos &amp;&amp; (SA[i-1]&gt;pos)) || ((SA[i]&gt;pos &amp;&amp; SA[i-1]&lt;pos)))
{
ans=max(ans,LCP[i]);
}
}

pf(&quot;%d\n&quot;,ans);

return 0;
}

```