前端笔试 本地正确，牛客上错误

代码如下，有没有同学知道什么情况呀

const readline = require('readline')

const rl = readline.createInterface({

  input: process.stdin,

  output: process.stdout,

})

let now = 0

let a = 0

let b = 0

let flag = a.length > b.length

rl.on('line', function (line) {

  now++

  if (now === 1) {

    a = line.split('')

  } else {

    b = line.split('')

    flag = a.length > b.length

    if (a.length === b.length) {

      for (let i = 0; i < a.length; i++) {

        if (a[i] === b[i]) continue

        flag = a[i] > b[i]

        break

      }

    }

    let result = ''

    while (a.length) {

      let tmp = parseInt(a.pop()) - parseInt(b.pop()|| 0)

      if (tmp > 0) {

        result = result + tmp

      } else {

        result = tmp + 10 + result

        a[a.length - 1]--

      }

    }

    console.log(result)

  }

})