Fast Power

Calculate the a^n % b where a, b and n are all 32bit integers.

Example

For 2^31 % 3 = 2

For 100^1000 % 1000 = 0

分析：

利用公式：

(a * b) % p = (a % p * b % p) % p
a^n % b = (a^(n/2) * a^(n/2) * (a)) %b = ((a^(n/2) * a^(n/2))%b * (a)%b) %b = ((a^(n/2)%b * a^(n/2)%b)%b * (a)%b) %b

 class Solution {
     /*
      * @param a, b, n: 32bit integers
      * @return: An integer
      */
     public int fastPower(int a, int b, int n) {

         if (a == ) return ;
         if (n == ) return  % b;
         if (n ==  || a == ) return a % b;

         long temp =  fastPower(a, b, n / );
         temp = temp * temp;
         temp = temp % b;
         if (n %  == ) {
             temp = temp * (a % b);
             return (int)(temp % b);
         } else {
             return (int)temp;
         }
     }
 };