Multiply Strings

Description

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"

Note:

1. The length of both num1 and num2 is < 110.
2. Both num1 and num2 contain only digits 0-9.
3. Both num1 and num2 do not contain any leading zero, except the number 0 itself.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

Solution(javascript)

const addStrings = (num1, num2) => {
  const reversedNum1 = num1.split('').reverse()
  const reversedNum2 = num2.split('').reverse()
  const { shorter, longer } = reversedNum1.length < reversedNum2.length
    ? { shorter: reversedNum1, longer: reversedNum2 }
    : { shorter: reversedNum2, longer: reversedNum1 }

  let rem = 0
  let result = longer.reduce((acc, digit1, index) => {
    const total = parseInt(digit1, 10) + (parseInt(shorter[index], 10) || 0) + rem
    if (total >= 10) {
      rem = 1
    } else {
      rem = 0
    }
    return [
      ...acc,
      total >= 10 ? total - 10 : total,
    ]
  }, [])
  result = rem === 1 ? [...result, rem] : result
  return result.reverse().join('')
}

const zero = (count) => {
  let result = ''
  while (count >= 1) {
    result += '0'
    count -= 1
  }
  return result
}
const multiply = (num1 = '', num2 = '') => {
  let result = 0
  for (let i = num2.length - 1; i >= 0; i--) {
    let current = ''
    let base = 0
    for (let j = num1.length - 1; j >= 0; j--) {
      let currentResult = parseInt(num1[j], 10) * parseInt(num2[i], 10) + base
      if (currentResult >= 10) {
        base = Math.floor(currentResult / 10)
        currentResult -= base * 10
      } else {
        base = 0
      }
      current = `${currentResult + current}`
    }
    if (base > 0) {
      current = base + current
    }
    // 偷懒
    result = addStrings(result || '0', current + zero(num2.length - 1 - i))
  }
  if (result.length > 0 && result[0] === '0') {
    return '0'
  }
  return result
}