二分法优化leftpad函数性能（算法200题-37）

const leftPad = (str, length, target) => {
  let len = length - str.length,
    resStr = ''
  while (true) {
    // 判断 len % 2 === 1
    if (len & 1) {
      resStr += target
    }
    if (len === 1) {
      return resStr + str
    }
    target += target
    // parseInt(len / 2 )
    len = len >> 1
  }
}
leftPad('asdasd', 100, 'o')