function sub(num1, num2) {

            if(num1 === num2) return '0'

            function lt(num1, num2) {

                if (num1.length < num2.length) {

                    return true

                } else if (num1.length === num2.length) {

                    return num1 < num2

                } else {

                    return false

                }

            }

            let isMinus = false

            if (lt(num1, num2)) {

                [num1, num2] = [num2, num1]

                isMinus = true

            }

            let len = Math.max(num1.length, num2.length)

            num1 = num1.padStart(len, 0)

            num2 = num2.padStart(len, 0)

            let flag = 0,

                result = '',

                temp

            for (let i = len - 1; i >= 0; i--) {

                temp = parseInt(num1[i]) - flag - parseInt(num2[i])

                if (temp < 0) {

                    result = (10 + temp) + result

                    flag = 1

                } else {

                    result = temp + result

                    flag = 0

                }

            }

            result = (isMinus ? '-' : '') + result.replace(/^0+/, '')

            return result

        }

        // 传入字符串格式

        console.log(sub('9999999999011112','9999999999011111'

)) // 1

    console.log(sub('9999999999011112','9999999999011113'

)) // -1