[leetcode] Multiply Strings

https://leetcode.com/problems/multiply-strings/

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

public class Solution {
    public String multiply(String num1, String num2) {
        if("0".equals(num1) || "0".equals(num2)) {
            return "0";
        }
        int l1 = num1.length(), l2 = num2.length();
        int idx = l1+l2, carry = 0;
        char[] ans = new char[idx];
        Arrays.fill(ans, '0');
        
        for(int i=l1-1; i>=0; --i) {
            for(int j=l2-1; j>=0; --j) {
                int n = ans[idx-l2+j]-'0'+(num1.charAt(i)-'0')*(num2.charAt(j)-'0')+carry;
                ans[idx-l2+j] = (char)(n%10+'0');
                carry = n/10;
            }
            if(carry > 0) {
                ans[idx-l2-1] = (char)(ans[idx-l2-1]+carry);
                carry = 0;
            }
            --idx;
        }
        idx = 0;
        while('0' == ans[idx]) ++idx;
        return new String(ans, idx, l1+l2-idx);
    }
}