Problem Statement

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.

Problem Link

Multiply Strings - LeetCode

Reference Video

https://www.youtube.com/watch?v=5NdhK3tZViQ

Code

class Solution {
public:
  
    string multiply(string num1, string num2) {
        
        if(num1=="0"||num2=="0")
            return "0";
        
        int l1 = num1.size();
        int l2 = num2.size();
        
        vector<int> res(l1+l2,0);
        
        int i = l2-1;
        int pf = 0;//power factor
        
        while(i>=0)
        {
            int ival = num2[i] - '0';
            int carry = 0;
            int k = res.size() - 1 - pf;
            int j = l1-1;
            
            while(j>=0 || carry)
            {
                int jval;
                if(j>=0)
                    jval = num1[j]-'0';
                else
                    jval = 0;
                
                int prod = ival*jval + carry + res[k];
               
                res[k] = prod%10;
                carry = prod/10;
                
                j--;
                k--;
                    
                
            }
            
            pf++;
            i--;
            
        }
        
        string ans = "";
        
        for(int val:res)
        {
            ans+=to_string(val);
        }
        
        //Managing leading 0s
        int in = 0;
        int jn = ans.size();
        for(;in<ans.size();in++)
        {
            if(ans[in]!='0')
                break;
        }
        
        return ans.substr(in,jn-in+1);
        
        
    }
};