function sumString(num1, num2) {

if (!num1 && !num2) return '0'

if (!num1) return num2

if (!num2) return num1

let carry = 0

let resStack = []

let leftStack = num1.split('')

let rightStack = num2.split('')

while (leftStack.length || rightStack.length) {

let leftVal = leftStack.length ? parseInt(leftStack.pop(), 10) : 0

let rightVal = rightStack.length ? parseInt(rightStack.pop(), 10) : 0

let sum = leftVal + rightVal + carry

carry = Math.floor(sum / 10)

resStack.push(sum % 10)

}

if (carry > 0) {

resStack.push(carry)

}

return resStack.reverse().join('')

}

function simpleMutiply(num1, num2) {

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

num2 = parseInt(num2, 10)

if (!num2) return 0

let carry = 0

let resStack = []

for (let i = num1.length - 1; i > -1; i--) {

let sum = parseInt(num1[i]) * num2 + carry

carry = Math.floor(sum / 10)

resStack.push(sum % 10)

}

if (carry > 0) {

resStack.push(carry)

}

return resStack.reverse().join('')

}

function patchZero(num1, count) {

if (!num1) return 0

for (let i = 0; i < count; i++) {

if (parseInt(num1, 10)) num1 += '0'

}

return num1

}

var multiply = function(num1, num2) {

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

let res = '0'

for (let i = num2.length - 1; i > -1; i--) {

let curSum = simpleMutiply(num1, num2[i])

res = sumString(res, patchZero(curSum, num2.length - i -1))

}

return res

};