C++ :

#include <cstdio>
#include <cstring>
#include <cstdlib>
using namespace std;

struct node
{
	int x,y;
}p[1001][1001];

char s1[1100], s2[1100],s[1100];

int f[1100][1100],len1,len2;

int mymax ( int x, int y ){ return x > y ? x : y; }

int main ()
{
	
    scanf ( "%s", s1 + 1 ); len1 = strlen ( s1 + 1 );
    scanf ( "%s", s2 + 1 ); len2 = strlen ( s2 + 1 );
    memset(f,0,sizeof(f));
    
    for (int i = 1; i <= len1; i ++ )
	{
        for (int j = 1; j <= len2; j ++ )
		{
            if ( s1[i] == s2[j] )
			{
                f[i][j] = f[i - 1][j - 1] + 1;
                p[i][j].x=i;
                p[i][j].y=j;
            }
            else
			{
                if( f[i-1][j]  >  f[i][j-1] )
                {
                	f[i][j]=f[i-1][j];
                	p[i][j]=p[i-1][j];
                }
                else
                {
                	f[i][j]=f[i][j-1];
                	p[i][j]=p[i][j-1];
                }
            }
        }
    }
    printf ( "%d\n", f[len1][len2] );
    
    int x=len1,y=len2,len=0;
    
    while( f[x][y]>0)
    {
    	int xx,yy;
    	xx=x;yy=y;
    	s[++len]=s1[   p[xx][yy].x   ];// s[++len]=s2[ p[xx][yy].y];
    	x=p[xx][yy].x-1;
    	y=p[xx][yy].y-1;
    }
    for(int i=len;i>=1;i--) printf("%c",s[i]);
    printf("\n");
    return 0;
}