Largest palindrome product - euler project

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广开言路
广开言路 2020-12-10 22:05

I was trying to solve project Euler problem 4 which is:

A palindromic number reads the same both ways. The largest palindrome made from the product of tw

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  •  爱一瞬间的悲伤
    2020-12-10 22:41

    This method is significantly faster than previous methods. It starts by evaluating 999 * 999. It is an implementation of the method proposed in Puzzled over palindromic product problem

    We would like to try larger products before smaller products, so next try 998 * 998, with the outer loop decreasing by one each time. In the inner loop, take the outer limit number to create (n+y)(n-y) (which is always less than n^2), iterating over y until one of the factors is too large or too small.

    From https://pthree.org/2007/09/15/largest-palindromic-number-in-python/, one of the factors must be a multiple of 11. Check to see if one of the factors is a multiple of 11 and that the product is greater than the previously found (or initial) palindromic number.

    Once these tests are satisfied, see if the product is a palindrome.

    Once a palindrome is found, we can raise the limit on the outer loop to the square root of the palindrome, since that is the minimum value that could possibly be an answer.

    This algorithm found the answer in only 475 comparisons. This is far better than 810,000 proposed by the simple methods, or even 405450.

    Can anyone propose a faster method?

    Longest palindromes:
    Max factor   Max Palindrome
    9999         99000099
    99999        9966006699
    999999       999000000999
    9999999      99956644665999
    99999999     9999000000009999
    999999999    999900665566009999
    
    public class LargestPalindromicNumberInRange {
        private final long lowerLimit;
        private final long upperLimit;
        private long largestPalindrome;
        private long largestFirstFactor;
        private long largestSecondFactor;
        private long loopCount;
        private long answerCount;
    
    public static void main(String[] args) {
        long lowerLimit = 1000;
        long upperLimit = 9999;
        LargestPalindromicNumberInRange palindromicNumbers = 
                new LargestPalindromicNumberInRange(lowerLimit, upperLimit);
        palindromicNumbers.TopDown();
    }
    
    private LargestPalindromicNumberInRange(long lowerLimit, long upperLimit){
        this.lowerLimit = lowerLimit;
        this.upperLimit = upperLimit;
    }
    private void TopDown() {
        loopCount = 0;
        answerCount = 0;
        largestPalindrome = lowerLimit * lowerLimit;
        long initialLargestPalindrome = largestPalindrome;
        long lowerFactorLimit = lowerLimit;
        for (long limit = upperLimit; limit > lowerFactorLimit; limit--){
            for (long firstFactorValue = limit; firstFactorValue >= limit - 1; firstFactorValue--) {
                long firstFactor = firstFactorValue;
                long secondFactor = limit;
                while(secondFactor <= upperLimit && firstFactor >= lowerLimit){
                    if (firstFactor % 11 == 0 || secondFactor % 11 == 0) {
                        long product = firstFactor * secondFactor;
                        if (product < largestPalindrome) { break; }
                        loopCount++;
                        if (IsPalindromic(product)) {
    //                    System.out.print("Answer: " + product + "\n");
                            answerCount++;
                            largestPalindrome = product;
                            largestFirstFactor = firstFactor;
                            largestSecondFactor = secondFactor;
                            lowerFactorLimit = (long) Math.sqrt(largestPalindrome);
                            break;
                        }
                    }
                    firstFactor--;
                    secondFactor++;
                }
            }
            System.out.print("Answer: " + largestPalindrome + "\n");
            System.out.print("Factor1: " + largestFirstFactor + "\n");
            System.out.print("Factor2: " + largestSecondFactor + "\n");
            System.out.print("Loop count: " + loopCount + "\n");
            System.out.print("Answer count: " + answerCount + "\n");
        }
    private boolean IsPalindromic(Long x) {
        String forwardString = x.toString();
    
        StringBuilder builder = new StringBuilder();
        builder.append(forwardString);
        builder = builder.reverse();
        String reverseString = builder.toString();
    
        return forwardString.equals(reverseString);
    }
    

    }

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