Returns length of function f or longest common substring of X [ 0. . m - 1 ] and Y [ 0. . n - 1 ]
int lcs ( int i , int j , int count ) { if ( i == 0 j == 0 ) return count ; if ( X [ i - 1 ] == Y [ j - 1 ] ) { count = lcs ( i - 1 , j - 1 , count + 1 ) ; } count = max ( count , max ( lcs ( i , j - 1 , 0 ) , lcs ( i - 1 , j , 0 ) ) ) ; return count ; }