LeetCode 43. Multiply Strings (javascript)

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

Note: You must not use any built-in BigInteger library or convert the inputs to integer directly.

Example 1:

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

Example 2:

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

Constraints:

1 <= num1.length, num2.length <= 200
num1 and num2 consist of digits only.
Both num1 and num2 do not contain any leading zero, except the number 0 itself.

Solution:

/**
 * @param {string} num1
 * @param {string} num2
 * @return {string}
 */
var multiply = function(num1, num2) {
    let len1 = num1.length;
    let len2 = num2.length;
    let carry = 0;
    let val = 0;
    let index = 0;
    let res = Array(len1 + len2).fill(0);
    
    for (let i = len1 - 1; i >= 0; i--) {
        carry = 0;
        for (let j = len2 - 1; j >= 0; j--) {
            // calucate from the right position and store in result array from index 0
            // reverse it later
            index = len1 + len2 - 2 - i - j;
            // basic math calculation
            val = (num1[i] * num2[j]) + res[index] + carry;
            carry = Math.floor(val / 10);
            res[index] = val % 10;
        }
        // check if has carry 1
        if (carry) res[index + 1] = carry;
    }
    
    // before reverse, remove the begining 0
    while (res.length > 1 && res[res.length - 1] === 0) res.pop();
    return res.reverse().join("");
};