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  1. class ProjectEulerQ04 {
  2. 	private static int reverse(int n){
  3. 		int reversed = 0;
  4. 		while(n>0){
  5. 			reversed *= 10;
  6. 			reversed += n%10;
  7. 			n /= 10;
  8. 		}
  9. 		return reversed;
  10. 	}
  11. 	public static void method1(){
  12. 		//checks x*y where 1<x<=y<=999
  13. 		int max=0;
  14. 		int candidate;
  15. 		for(int x=1;x<=999;x++){
  16. 			for(int y=x;y<=999;y++){
  17. 				candidate = x*y;
  18. 				if(candidate == reverse(candidate)){
  19. 					if(candidate > max){
  20. 						max = candidate;
  21. 					}
  22. 				}
  23. 			}
  24. 		}
  25. 		System.out.println("Output: "+max);
  26. 	}
  27. 	public static void method2(){
  28. 		//builds table of x*y, where 1 < y <= x <= 999
  29. 		boolean[] is_product = new boolean[1000000];
  30. 		for(int x=1;x<=999;x++){
  31. 			for(int y=1;y<=x;y++){
  32. 				is_product[x*y] = true;
  33. 			}
  34. 		}
  35. 		//checks all palindromes
  36. 		int max = 0;
  37. 		int power = 1;
  38. 		int temp1,temp2;
  39. 		for(int i=1;i<=999;i++){
  40. 			if(reverse(i) == 1){
  41. 				power *= 10;
  42. 			}
  43. 			temp1 = (i/10)*(power)+reverse(i);
  44. 			temp2 = i*(power)+reverse(i);
  45. 			if(is_product[temp1]){
  46. 				if(temp1 > max){
  47. 					max = temp1;
  48. 				}
  49. 			}
  50. 			if(is_product[temp2]){
  51. 				if(temp2 > max){
  52. 					max = temp2;
  53. 				}
  54. 			}
  55. 		}
  56. 		System.out.println("Output: "+max);
  57. 	}
  58. 	public static void main(String args[]){
  59. 		long start;
  60.  
  61. 		System.out.println("Solve by checking each product:");
  62. 		start = System.currentTimeMillis();
  63. 		method1();
  64. 		System.out.println("Time used: "+(System.currentTimeMillis()-start)+" ms");
  65.  
  66. 		System.out.println();
  67.  
  68. 		System.out.println("Solve by checking each palindrome:");
  69. 		start = System.currentTimeMillis();
  70. 		method2();
  71. 		System.out.println("Time used: "+(System.currentTimeMillis()-start)+" ms");
  72. 	}
  73. }
Success #stdin #stdout 0.21s 320512KB
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Solve by checking each product:
Output: 906609
Time used: 102 ms

Solve by checking each palindrome:
Output: 906609
Time used: 6 ms
https://ideone.com/PqNHxp
language:
Java (HotSpot 12)
created:
7 years ago
visibility:
public

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