Powxn.cpp

/*
Pow(x, n)
=========

Implement pow(x, n), which calculates x raised to the power n (i.e., xn).

Example 1:
Input: x = 2.00000, n = 10
Output: 1024.00000

Example 2:
Input: x = 2.10000, n = 3
Output: 9.26100

Example 3:
Input: x = 2.00000, n = -2
Output: 0.25000
Explanation: 2-2 = 1/22 = 1/4 = 0.25

Constraints:
-100.0 < x < 100.0
-231 <= n <= 231-1
-104 <= xn <= 104
*/

class Solution
{
public:
  double myPow(double x, int n)
  {
    long long N = abs(n);
    double ans = 1.0;
    while (N > 0)
    {
      if (N % 2 == 1)
      {
        ans = ans * x;
        N--;
      }
      else
      {
        x = x * x;
        N = N / 2;
      }
    }

    if (n < 0)
      return 1.0 / ans;
    else
      return ans;
  }
};