Multiply Strings Leetcode 43 question Correct Solution

 Question :

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

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

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.

/**

 * @param {string} num1

 * @param {string} num2

 * @return {string}

 */

var add = function(a, b){

    // length of a should be smaller

    if(a.length > b.length)return add(b, a);

    // iterate a

    let i = a.length-1, j = b.length-1, carry = 0, currAns='';

    while(i >= 0){

        // get nums

        let numA = a[i]-0;

        let numB = b[j]-0;

        // add numA and numB + carry

        let sum = numA + numB + carry;

        sum = '' + sum;

        // add ones digit to currAns

        currAns = (sum.length > 1) ? sum[1] + currAns : sum + currAns;

        // take carry if any

        carry = (sum.length > 1) ? 1 : 0;

        // decrement i and j

        i--;

        j--;

    }

    // if b was large number i.e j != 0

    while(j >= 0){

       

            let numB = b[j]-0;

            // add numB + carry

            let sum = numB + carry;

            sum = '' + sum;

            // add ones digit to currAns

            currAns = (sum.length > 1) ? sum[1] + currAns : sum + currAns;

            // take carry if any

            carry = (sum.length > 1) ? 1 : 0;

            // decrement j

            j--;  

    }

    // check if there is a carry remaining to be settled

    if(carry != 0){

        currAns = '1' + currAns;

    }

    return currAns;

}

var sumOfArr = function(arr){

    if(arr.length == 1)return arr[0];

    // ans = sum of first two numbers;

    let ans = add(arr[0], arr[1]+'0');

    // start iterating from the third number;

    let i = 2;

    let zero = '0';

    while(i < arr.length){

        let b = arr[i]+zero.repeat(i)

        ans = add(ans, b);

        i++;

    }

    return ans;

}

var multiply = function(num1, num2) {

    if(num2.length > num1.length)return multiply(num2, num1);

    if(num1-0 === 0 || num2-0 === 0)return "0";

    let i = num2.length-1;

    // array to sum and get final ans

    let sumArr = [];

    // num1 * num2 = ans

    // iterating num2 from ones place

    while(i >= 0){

        let currAns = '', carry = 0, ans;

        // Convert num2 char to int

        let num = num2[i]-0;

        let j = num1.length-1;

        // iterating num1 from ones place

        while(j >= 0){

            // convert num1 char to int

            let n = num1[j]-0;

            // (multiply num1 int to num2 int) + (carry if any) = ans

            ans = (n*num) + carry;

            // convert ans to string

            ans = ''+ans;

            // store ones digit of ans to currAns;

            currAns = (ans.length > 1) ? ans[1] + currAns : ans[0] +currAns;

            // store twos digit of ans (int) to carry if any;

            carry = (ans.length > 1) ? ans[0]-0 : 0;

            // if j == 0 And carry != 0 then push carry to the currAns;

            if(j == 0 && carry != 0)currAns = '' + carry + currAns;

            // decrement j

            j--;

        }

        // push currAns to sumArr

        sumArr.push(currAns);

        // decrement i;

        i--;

    }

    return sumOfArr(sumArr);

   

};

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