Multiply Strings

public class Solution {
    public String multiply(String num1, String num2) {
        if(num1==null || num2==null || num1.length()*num2.length()==0) return null;
        if(num1.charAt(0)=='0'||num2.charAt(0)=='0') return "0";
        String n1 = new StringBuilder(num1).reverse().toString();
        String n2 = new StringBuilder(num2).reverse().toString();
        int len = n1.length()+n2.length();
        int[] b = new int[len];
        for(int i=0;i<n1.length();i++){
            int p = n1.charAt(i)-'0';
            for(int j=0; j<n2.length();j++){
                int q = n2.charAt(j)-'0';
                b[i+j] += p*q;
            }
        }
        StringBuilder t = new StringBuilder();
        int car = 0;
        for(int i=0; i<len;i++){
            int dig = (b[i]+car)%10;
            car = (b[i]+car)/10;
            t.insert(0,dig);
        }
        while(t.charAt(0)=='0' && t.length()>1){
            t.deleteCharAt(0);
        }
        return t.toString();
    }
}

ref http://www.cnblogs.com/springfor/p/3889706.html