Multiply Strings

Source

• leetcode: 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.

Java

public class Solution {
    public String multiply(String num1, String num2) {
        num1 = new StringBuilder(num1).reverse().toString();
        num2 = new StringBuilder(num2).reverse().toString();

        int[] d = new int[num1.length() + num2.length()];
        for(int i=0; i<num1.length(); i++){
            int a = num1.charAt(i)-'0';
            for(int j=0; j<num2.length(); j++){
                int b = num2.charAt(j)-'0';
                d[i+j] += a*b;
            }
        }

        StringBuilder sb = new StringBuilder();
        for(int i=0; i<d.length; i++){
            int digit = d[i]%10;
            int carry = d[i]/10;
            sb.insert(0, digit);
            if(i<d.length-1){
                d[i+1]+=carry;
            }
        }
        //trim leading zeros
        while(sb.length()>0 && sb.charAt(0)=='0'){
            sb.deleteCharAt(0);
        }
        return sb.length() == 0 ? "0" : sb.toString();
    }
}

Reference

Multiply Strings | 大整数的字符串乘法