Calculate the a^n % b where a, b and n are all 32bit integers.Example
For 2^31 % 3 = 2For 100^1000 % 1000 = 0Challenge
O(logn)

 1 class Solution {
 2     /*
 3      * @param a, b, n: 32bit integers
 4      * @return: An integer
 5      */
 6     public int fastPower(int a, int b, int n) {
 7         // write your code here
 8         long ret = getPower(a, b, n);
 9         return (int)ret;
10     }
11     public long getPower(int a, int b, int n){
12         if(a == 0) return 0;
13         if(n == 0) return 1 % b;
14         if(n == 1) return a % b;
15
16         long ret = getPower(a, b, n/2);
17         ret *= ret;
18         ret %= b;
19         if(n % 2 == 1){
20             ret = ret * (a % b);
21         }
22         return ret % b;
23     }
24 };

 1 class Solution {
 2     public int fastPower(int a, int b, int n) {
 3         if (n == 0) return 1%b;
 4         long half = fastPower(a, b, n/2);
 5         if (n % 2 == 0) {
 6             long temp = half * half;
 7             return (int)(temp % b);
 8         }
 9         else {
10             long temp = (half * half)%b * (a%b);
11             return (int)(temp % b);
12         }
13     }
14 };

 1 class Solution {
 2     /*
 3      * @param a, b, n: 32bit integers
 4      * @return: An integer
 5      */
 6     public int fastPower(int a, int b, int n) {
 7         long ret = helper(a, b, n);
 8         return (int)ret;
 9     }
10
11     public long helper(int a, int b, int n) {
12         if (n == 0) return 1%b;
13         long half = helper(a, b, n/2);
14         if (n % 2 == 0) {
15             return half * half % b;
16         }
17         else {
18             return ((half * half % b) * a%b)%b;
19         }
20     }
21 };