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  1. #include <stdio.h>
  2. #include <math.h>
  3.  
  4. /* A palindromic number reads the same both ways. The largest palindrome made
  5.  * from the product of two 2-digit numbers is 9009 = 91 × 99.
  6.  *
  7.  * Find the largest palindrome made from the product of two 3-digit numbers.
  8. */
  9.  
  10. unsigned reverse(unsigned i)
  11. {
  12.     unsigned r = 0;
  13.     while (i)
  14.     {
  15.         r *= 10;
  16.         r += i % 10;
  17.         i /= 10;
  18.     }
  19.  
  20.     return r;
  21. }
  22.  
  23. int main()
  24. {
  25.     unsigned largest = 0;
  26.     for (unsigned j = 999; j>99; --j)
  27.     {
  28.         for (unsigned k = j; k>99; --k) 
  29.         {
  30.             const unsigned prod = j*k;
  31.  
  32.             //printf("%d x %d = %d\n", j, k, prod);
  33.  
  34.             const unsigned lhs = prod / 1000;
  35.             const unsigned rhs = prod % 1000;
  36.  
  37.             if (lhs==reverse(rhs) && prod>largest)
  38.                 largest = prod;
  39.         }
  40.     }
  41.  
  42.     printf("%d\n", largest);
  43.     return 0;
  44. }
  45.  
Success #stdin #stdout 0s 3340KB
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https://ideone.com/0PpOs8
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
secret

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