231. Power of Two

原题：231. Power of Two

方法一

思路：求底，判断结果是否是整数

// Runtime: 88 ms, faster than 18.54% of JavaScript online submissions for Power of Two.
// Memory Usage: 36.6 MB, less than 15.38% of JavaScript online submissions for Power of Two.
var isPowerOfTwo = function(n) {
    let result = Math.log2(n);
    return ~~result == result;
};

方法二

思路：位运算，因为2的指数结果二进制一定是以1开头，剩下位数都是0的串。

// Runtime: 96 ms, faster than 15.25% of JavaScript online submissions for Power of Two.
// Memory Usage: 36.4 MB, less than 15.38% of JavaScript online submissions for Power of Two.
var isPowerOfTwo = function (n) {
  return n > 0 && !(n & (n - 1));//2为底的任意次方都一定>0
};

test cases

test("test1", () => {
  expect(isPowerOfTwo(1)).toEqual(true);
});

test("test2", () => {
  expect(isPowerOfTwo(16)).toEqual(true);
});

test("test3", () => {
  expect(isPowerOfTwo(218)).toEqual(false);
});

test("test4", () => {
  expect(isPowerOfTwo(-2147483648)).toEqual(false);
});