Answer by ggf31416 for Quickest Method to Reverse in String in C#.net

A you want to compare a number with its reverse it may be faster to reverse the number using division rather than converting it to a string. I still need to test the speed of it.

 private static int Reverse(int num) {     int res = 0;     while (num > 0) {        int rm ;        num = Math.DivRem(num, 10, out rm);        res = res * 10 + rm;     }     return res;  }

EDIT:DivRem was about 1% faster than division and module in my computer.A speed optimization is exit if the last digit is 0:

  private static int Reverse(int num) {     int res = 0;     int rm;     num = Math.DivRem(num, 10, out rm);     //Some magic value or return false, see below.     if (rm == 0) return -1 ;      res = res * 10 + rm;     while (num > 0) {        num = Math.DivRem(num, 10, out rm);        res = res * 10 + rm;     }     return res ;  }

Making the method return a bool was slightly slower than comparing to a bool in a loop in my computer, but I don't understand why. Please test in your computer.

Multiplication and bit-shifing should be faster than division but probably are not precise enough. EDIT: using long seems be precise enough.

  private static int FastReverse(int num) {     int res = 0;     int q = (int)((214748365L * num) >> 31);     int rm = num - 10 * q;     num = q;     if (rm == 0) return -1;     res = res * 10 + rm;     while (num > 0) {        q = (int)((214748365L * num) >> 31);        rm = num - 10 * q;        num = q;        res = res * 10 + rm;     }     return res;  }

(214748365L * num) >> 31 is equal to i / 10 until 1,073,741,829 where 1 / 10 gives 107374182 and the multiplication + binary shifting gives 107374183.