29. Implement Pow

The problem can be found at the following link: Problem Link

Problem Description

You are given a floating point number b and an integer e, and your task is to calculate ( b^e ) (b raised to the power of e).

Examples:

Input: b = 3.00000, e = 5 Output: 243.00000

Input: b = 0.55000, e = 3 Output: 0.16638

Input: b = -0.67000, e = -7 Output: -16.49971

Constraints:

• -100.0 < b < 100.0

• -109 <= e <= $10^9$

• Either b is not zero or e > 0.

• -104 <= be <= $10^4$

My Approach

1. Optimized Binary Exponentiation: The problem can be efficiently solved using the binary exponentiation approach (also known as exponentiation by squaring). This method reduces the number of multiplications required to compute ( b^e ) by breaking the problem down recursively.

2. Steps:

   • If e == 0, return 1, as any number raised to the power of 0 is 1.

   • If e is negative, calculate the result using $( \frac{1}{b^e} )$.

   • For positive e, repeatedly divide e by 2, multiplying b by itself for each halving of e.

   • The result is obtained by squaring the value of b for each step and multiplying it with the current result when necessary.

3. Optimization: Using a while loop or recursion, the approach reduces the time complexity from O(e) (in the naive approach) to O(log e) , which is significantly faster for large values of e.

Time and Auxiliary Space Complexity

• Expected Time Complexity: $O(log |e|)$, where $(e)$ is the exponent. The exponent is reduced by half in each iteration.

• Expected Auxiliary Space Complexity: $O(1)$, as we only use a constant amount of additional space.

Code (C++)

class Solution {
public:
    double power(double b, int e) {
        if (e == 0) return 1.0;
        double half = power(b, e / 2);
        return e % 2 == 0 ? half * half : (e > 0 ? b * half * half : (1.0 / b) * half * half);
    }
};

👨‍💻 Alternative Approaches

1. Iterative Method (Binary Exponentiation)

class Solution {
public:
    double power(double b, int e) {
        double result = 1.0;
        long long exp = abs((long long)e);
        while (exp > 0) {
            if (exp % 2 == 1) result *= b;
            b *= b;
            exp /= 2;
        }
        return e < 0 ? 1.0 / result : result;
    }
};

• Optimization: No recursion, reduced overhead.

• Time Complexity: $( O(\log e) )$

• Space Complexity: $( O(1) )$

2. Tail-Recursive Method

class Solution {
public:
    double power(double b, int e, double result = 1.0) {
        if (e == 0) return result;
        if (e < 0) return power(1.0 / b, -e, result);
        return e % 2 == 0 ? power(b * b, e / 2, result) : power(b * b, e / 2, result * b);
    }
};

• Optimization: Tail recursion ensures no stack buildup in compilers with tail-call optimization.

• Time Complexity: $( O(\log e) )$

• Space Complexity: $( O(\log e) )$ (if no tail-call optimization)

3. Using Built-in Function

// #include <cmath>
class Solution {
public:
    double power(double b, int e) {
        return std::pow(b, e);
    }
};

• Optimization: Leverages highly optimized library implementation.

• Time Complexity: $( O(1) )$ (Library-optimized)

• Space Complexity: $( O(1) )$

4. Modified Iterative Approach (Handling Edge Cases)

class Solution {
public:
    double power(double b, int e) {
        long long exp = e;
        double result = 1.0;
        if (exp < 0) { b = 1.0 / b; exp = -exp; }
        while (exp) {
            if (exp & 1) result *= b;
            b *= b;
            exp >>= 1;
        }
        return result;
    }
};

• Optimization: Uses bitwise operations and handles edge cases like negative exponents directly.

• Time Complexity: $( O(\log e) )$

• Space Complexity: $( O(1) )$

Summary of Approaches:

Approaches	Time Complexity	Space Complexity	Notes
Recursive	$O(\log e)$	$O(\log e)$	Simple, uses recursion.
Iterative (Binary Exp.)	$O(\log e) $	$O(1)$	Most efficient approach.
Tail-Recursive	$O(\log e) $	$O(\log e)$	Requires tail-call opt.
Built-in std::pow	$O(1) $	$O(1)$	Leveraging library power.
Modified Iterative	$O(\log e) $	$O(1)$	Handles edge cases well.

The iterative binary exponentiation is typically the best choice for performance-critical scenarios.

Code (Java)

class Solution {
    public double power(double b, int e) {
        if (e == 0) return 1.0;
        double half = power(b, e / 2);
        return e % 2 == 0 ? half * half : (e > 0 ? b * half * half : (1.0 / b) * half * half);
    }
}

Code (Python)

class Solution:
    def power(self, b: float, e: int) -> float:
        result = 1.0
        exp = abs(e)
        while exp > 0:
            if exp % 2 == 1:
                result *= b
            b *= b
            exp //= 2
        return result if e >= 0 else 1.0 / result

Contribution and Support

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