PowXN50.java

/**
 * Implement pow(x, n), which calculates x raised to the power n (xn).
 * 
 * Example 1:
 * Input: 2.00000, 10
 * Output: 1024.00000
 * 
 * Example 2:
 * Input: 2.10000, 3
 * Output: 9.26100
 * 
 * Example 3:
 * Input: 2.00000, -2
 * Output: 0.25000
 * Explanation: 2-2 = 1/22 = 1/4 = 0.25
 * 
 * Note:
 * -100.0 < x < 100.0
 * n is a 32-bit signed integer, within the range [−231, 231 − 1]
 */

public class PowXN50 {
    /**
     * https://leetcode.com/problems/powx-n/solution/
     */
    public double myPow(double x, int n) {
        long N = n;
        if (N < 0) {
            x = 1 / x;
            N = -N;
        }
        return fastPow(x, N);
    }

    private double fastPow(double x, long n) {
        if (n == 0) {
            return 1.0;
        }
        double half = fastPow(x, n / 2);
        if (n % 2 == 0) {
            return half * half;
        } else {
            return half * half * x;
        }
    }


    /**
     * https://leetcode.com/problems/powx-n/solution/
     */
    public double myPow2(double x, int n) {
        long N = n;
        if (N < 0) {
            x = 1 / x;
            N = -N;
        }
        double ans = 1;
        double current_product = x;
        for (long i = N; i > 0; i /= 2) {
            if ((i % 2) == 1) {
                ans = ans * current_product;
            }
            current_product = current_product * current_product;
        }
        return ans;
    }

}