实现 pow(x, n) ，即计算 x 的整数 n 次幂函数（即，x ** n ）。

思路

  1. x ** (a + b) = (x ** a) * (x ** b)
  2. x ** (a * b) = (x ** a) ** b
  3. x ** (2k + i) = (x ** i) * (x ** 2k) = (x ** i) * ((x ** 2) ** k)
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var myPow = function (x, n) {
  const isNegative = n < 0
  n = Math.abs(n)
  const factors = []
  while (n > 0) {
    factors.push(n % 2 === 1 ? x : 1)
    n = Math.floor(n / 2)
    x = x * x
  }
  const product = factors.reduce((result, cur) => result * cur, 1)
  return isNegative ? 1 / product : product
}