50. Pow(x, n)

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]

 

AC code:

Recursive:

class Solution {
public:
    double myPow(double x, int n) {
        if (n >= 0) return pow(x, n);
        else return 1.0 / pow(x, n);
    }
    
    double pow(double x, int n) {
        if (n == 0) {
            return 1;
        }
        double y = pow(x, n / 2);
        if (n % 2 == 0) {
            return y * y;
        } else {
            return y * y * x;
        }
    }
};

 

Runtime: 4 ms, faster than 99.40% of C++ online submissions for Pow(x, n).

 

Iteration:

class Solution {
public:
    double myPow(double x, int n) {
        long y = abs((long)n);
        double res = 1;
        while (y > 0) {
            if (y % 2 != 0) {
                res *= x;
            }
            x *= x;
            y /= 2;
        }
        if (n < 0) {
            res = 1.0 / res;
        }
        return res;
    }

};

Runtime: 4 ms, faster than 99.40% of C++ online submissions for Pow(x, n).