java - checking number is power of 2 for numbers greater than 2^64, implementation of manual division -

i trying check if number (input string) power of 2. problem number might greater 2^64 (long long limit). if store in double cannot binary operations on (so trick v & (v - 1) == 0 not work). here solution stumbled upon:

public class solution {     public int power(string a) {          string dividend = a;         stringbuilder str;          if (a == null || a.length() == 0)             return 0;          if (a.length() == 1 && a.charat(0) == '0')             return 0;          while (dividend.length() > 0 && !dividend.equalsignorecase("2")) {             str = new stringbuilder();             int carry = 0;             int n = dividend.length();              if (n > 0) {                 int num = dividend.charat(n - 1) - '0';                  if (num % 2 == 1)                     return 0;             }              (int = 0; < n; i++) {                  char c = (char) (dividend.charat(i) - '0');                 int res = c + 10 * carry;                  c = (char) (res / 2 + '0');                 carry = res % 2;                  str.append(c);             }              if (str.charat(0) == '0')                 str.deletecharat(0);              dividend = str.tostring();         }          return 1;      }  } 

i having problems understanding solution. first check done if number odd or not. having problems understanding implementation of manual division done in problem (especially arithmetic operations involving both char , int). '0' == 48 , subtracting '0' character converts integer value. can explain me how division operation has been implemented here? thanks!

the easiest way understand implementation pen , paper test. skip part understood , focus on last for loop.

let's take "50" example-input. loop steps through string character-wise, starting on left. therefore, first character processed '5'. know, 5 odd. means have "carry" next division. result of 5 / 2 2. 2 first digit of result in division. notice if reverse our steps, 2 * 2, 4. reason have carry on (the 1 missing, since 5 - 4 1). may notice that, if divide 2, can either carry 0 (nothing) or 1 (if , if dividend odd).

now on next digit, 0. have carry 1 last division on step. since moved 1 digit left, 1 previos step becomes 10 in step. moved "one order of magnitude down", therefore have multiply carryed part 10. means calculate (0 + (1 * 10)) / 2, 10 / 2 = 5. 5 second digit of result. since dividend even, need nothing carry over.

the final ouput therefore "25", expected.