7. 整数反转

解法一

/**
 * @param {number} x
 * @return {number}
 */
var reverse = function (x) {
  const MAX_VALUE = 2 ** 31 - 1
  const MIN_VALUE = -(2 ** 31)
  const signal = x < 0 ? -1 : 1
  const newX = Math.abs(x) + ''
  let result = newX.split('').reverse().join('')
  if (result > MAX_VALUE || result < MIN_VALUE) {
    return 0
  }
  return signal * result
}

解法二

/**
 * @param {number} x
 * @return {number}
 */
var reverse = function (x) {
  let res = 0
  while (x) {
    res = res * 10 + (x % 10)
    if (res > Math.pow(2, 31) - 1 || res < Math.pow(-2, 31)) return 0
    x = ~~(x / 10)
  }
  return res
}