JAVA Project Euler P. #8, Find greatest product from 1000 digit number

Question

https://projecteuler.net/problem=8 link to problem

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product.(number is in the code as String num)

The answer i am getting is "9205903071867879424" (wrong answer) Please point out the mistakes in my code and also suggest your solution to solve the problem efficiently as possible. I read the other threads about this problem but couldn't understand them and also there weren't enough efficient solutions.

public static void main(String[] args){
    String num = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
    long product=1;
    long greatestp=0;
    long limit=13;
    for (int i=0; i<num.length()-13; i++){
        product = 1;
        for (int j=0; j<limit; j++ ){
            product = (long) (product * (int) (num.charAt(j+i)));
        }
        if (greatestp<product){
            greatestp = product;
        }

    }
    System.out.println(greatestp);

}

P.S. I'm a beginner at JAVA, I would appreciate if you explain your solution in detail.

two observations that you can use to make optimizations: 1) stop multiplying when you run across a zero, and 2) if you have p = a * b * c *d, and you next want p = b * c * d * e, you can instead compute p = e * (p / a) — President James K. Polk, Jan 29 '17 at 15:04
I am getting your point, Can you post your solution with those optimizations? @JamesKPolk — KillerHawx, Jan 29 '17 at 15:14
@KillerHawx he cannot since the question is closed. There is a better place for asking for optimization: https://codereview.stackexchange.com - maybe there are already answers for that projecteuler problem on the site. — luk2302, Jan 29 '17 at 16:10

score 1 · Accepted Answer · answered Jan 29 '17 at 15:00

1

charAt gives you the ascii charcode, not the actual digit. Therefore the fastest way is to subtract the value of '0' from it:

product = product * (num.charAt(i+j) - '0');

answered Jan 29 '17 at 15:00

luk2302

55,258
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oh, thanks, i didn't know that, is there any way to get the actual digit from a string ? @luk2302 – KillerHawx Jan 29 '17 at 15:07
@KillerHawx since you have a `char` at hand, you can just do http://stackoverflow.com/questions/8172420/how-to-convert-a-char-to-a-string-in-java – luk2302 Jan 29 '17 at 15:08

JAVA Project Euler P. #8, Find greatest product from 1000 digit number

1 Answers1