multiply-strings.md

Multiply Strings [Medium]

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

Note:

The length of both num1 and num2 is < 110.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

Java solution

class Solution {
    
    public String multiply(String num1, String num2) {
        if (num1.equals("0") || num2.equals("0")) {
            return "0";
        }
        
        int n = num1.length();
        int m = num2.length();
        int length = n+m-1;
        int[] result = new int[length];
        
        for (int i = 0; i < n; i++) {
            for(int j =0; j < m; j++) {
                result[i+j] += (num1.charAt(n-i-1)-'0') * (num2.charAt(m-j-1)-'0');
            }
        }
        
        for(int i=0; i< length-1; i++) {
            result[i+1] += result[i]/10;
            result[i] = result[i] % 10;
        }
        int carry = result[length-1]/10;
        result[length-1] = result[length-1] % 10;
        
        StringBuilder sb = new StringBuilder();
        if (carry != 0) {
            sb.append(carry);
        }
        for (int i = result.length-1; i>-1; i--) {
            sb.append(result[i]);
        }
        return sb.toString();
    }
}