Multiply Strings，字符串相乘

问题描述：给定两个字符串，返回他们的乘积。

public class MultiplyStrings
{
	public String multiply(String num1, String num2)
	{
		String n1 = new StringBuilder(num1).reverse().toString();
		String n2 = new StringBuilder(num2).reverse().toString();

	    int[] d = new int[num1.length()+num2.length()];

	    //multiply each digit and sum at the corresponding positions
	    for(int i=0; i<n1.length(); i++){
	        for(int j=0; j<n2.length(); j++){
	            d[i+j] += (n1.charAt(i)-'0') * (n2.charAt(j)-'0');
	        }
	    }

	    StringBuilder sb = new StringBuilder();

	    //calculate each digit
	    for(int i=0; i<d.length; i++)
	    {
	        int mod = d[i]%10;
	        int carry = d[i]/10;
	        if(i+1<d.length)
	        {
	            d[i+1] += carry;
	        }
	        sb.insert(0, mod);
	    }

	    //remove front 0's
	    while(sb.charAt(0) == '0' && sb.length()> 1){
	        sb.deleteCharAt(0);
	    }

	    return sb.toString();
	}
}