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  1. def number2digits(long n) {
  2.     def result = []
  3.     for(/**/; n > 0; n /= 10) {
  4.         result.add(0, n % 10)
  5.     }
  6.     return (result) ? result : [0]
  7. }
  8. assert number2digits(123) == [1, 2, 3]
  9.  
  10. def digits2number(digits) {
  11.     return digits.inject(0l) { a, b -> a*10 + b }
  12. }
  13. assert digits2number([1, 2, 3]) == 123
  14.  
  15. def createPalindrome(long palindromeBeginning, boolean doubleDigitInMiddle) {
  16.     def palindromeBeginningDigits = number2digits(palindromeBeginning)
  17.     def palindromeEndingDigits = palindromeBeginningDigits.reverse()
  18.     if (!doubleDigitInMiddle) {
  19.         palindromeEndingDigits = palindromeEndingDigits.tail()
  20.     }
  21.     return digits2number(palindromeBeginningDigits + palindromeEndingDigits)
  22. }
  23. assert createPalindrome(123, false) == 12321
  24. assert createPalindrome(123, true) == 123321
  25.  
  26. COUNTER = 0
  27.  
  28. def findNDigitsDivisorPair(long x, int N) {
  29.     int MAX_DIVISOR = 10**N - 1
  30.     int MIN_DIVISOR = 10**(N-1)
  31.     int squareRoot = Math.sqrt(x)
  32.     if (MIN_DIVISOR <= squareRoot && squareRoot <= MAX_DIVISOR) {
  33.         int lowerDivisorBound = Math.max(x / MAX_DIVISOR as int, MIN_DIVISOR);
  34.         for (int i = lowerDivisorBound; i <= squareRoot; ++i) {
  35.             double quotient = (x as double) / i
  36.             ++COUNTER
  37.             //println "$x / $i = $quotient"
  38.             if (quotient % 1 == 0) return [i, quotient as int]
  39.         }
  40.     }
  41.     return null
  42. }
  43. assert findNDigitsDivisorPair(143, 2) == [11, 13]
  44.  
  45. def findBiggestDivisiblePalindrome(digitsCount) {
  46.     for (int begin = 10**digitsCount - 1; begin >= 10**(digitsCount-1); --begin) {
  47.         long palindrome = createPalindrome(begin, true)
  48.         def solution = findNDigitsDivisorPair(palindrome, digitsCount)
  49.         if (solution) {
  50.             println "Number of checks = " + COUNTER
  51.             println "$palindrome = ${solution[0]} * ${solution[1]}"
  52.             return palindrome
  53.         }
  54.     }    
  55. }
  56. //assert findBiggestDivisiblePalindrome(9) = 999996100001699999
  57.  
  58. def start = new Date().getTime()
  59. findBiggestDivisiblePalindrome(3)
  60. println "Execution time: ${new Date().getTime() - start} ms"
  61.  
Success #stdin #stdout 1.83s 332416KB
comments (?)
stdin 
Standard input is empty
stdout 
Number of checks = 2138
906609 = 913 * 993
Execution time: 179 ms
https://ideone.com/NzO3nu
language:
Groovy (groovy 2.5.6)
created:
8 years ago
visibility:
public

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