Fast Power

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

Analyse: divide and conquer. Be aware of overflow. 

Runtime: 12ms

class Solution {
public:
    /*
     * @param a, b, n: 32bit integers
     * @return: An integer
     */
    int fastPower(int a, int b, int n) {
        // write your code here
        if (!a || b == 1 || n < 0) return 0;
        if (!n && b != 1) return 1;
        if (n == 1) return a % b;
        
        long temp1 = fastPower(a, b, n / 2);
        long temp = temp1 * temp1 % b;
        if (n % 2) temp *= a % b;
        return (int)(temp % b);
    }
};