Power of two - Codewars - 不忘初心，上下求索。

题目

Write a function that determines if given number is a power of two. A power of two means a number of the form 2^n where n is an integer, i.e. the result of exponentiation with number two as the base and integer n as the exponent. I.e. 1024 is a power of two: it 2^10.

Example:

isPowerOfTwo(4096) // -> true

isPowerOfTwo(333)  // -> false

翻译：判断一个非负整数 n 是不是 2 的非负整数次幂（题目很绕）。

我的解法：

function isPowerOfTwo(n) {
  var tag = false;
  for (var i=0; i < n; i++) {
    if (Math.pow(2,i) == n ) {
      tag = true;
      break;
    }
  }
  return tag;
}

思路分析：依旧简单 ←_←，for 循环 n 次，让 2 的 i 次方与 n 对比

来看别人怎么写：

解法一：

function isPowerOfTwo(n) {
  return !(n & (n - 1));
}

解法二：

function isPowerOfTwo(n) {
  return Math.log(n) / Math.log(2) % 1 == 0;
}

解法三：

funcion isPowerOfTwo(n) {
  if (n < 2) return false;
  if (n == 2) return true;
  return isPowerOfTwo(n / 2);
}

解法四：

funcion isPowerOfTwo(n) {
  return /^10*$/.test(n.toString(2));
}

解法五（待定）：

funcion isPowerOfTwo(x) {
   return (
   x == 1 || x == 2 || x == 4 || x == 8 || x == 16 || x == 32 ||
   x == 64 || x == 128 || x == 256 || x == 512 || x == 1024 ||
   x == 2048 || x == 4096 || x == 8192 || x == 16384 ||
   x == 32768 || x == 65536 || x == 131072 || x == 262144 ||
   x == 524288 || x == 1048576 || x == 2097152 ||
   x == 4194304 || x == 8388608 || x == 16777216 ||
   x == 33554432 || x == 67108864 || x == 134217728 ||
   x == 268435456 || x == 536870912 || x == 1073741824 ||
   x == 2147483648);
}