字符串相乘

class Solution {
    public String multiply(String num1, String num2) {
        if(num1.equals("0")||num2.equals("0"))return "0";
        int len1=num1.length();
        int len2=num2.length();
        int[] num=new int[len1+len2];
        int product,i,j;
        for( i=len1-1;i>=0;i--){
            int carry=0;
            for( j=len2-1;j>=0;j--){
                product=carry+(int)(num1.charAt(i)-‘0‘)*(int)(num2.charAt(j)-‘0‘)+num[i+j+1];
                carry=product/10;
                num[i+j+1]=product%10;
            }
            num[i+j+1]=carry;
        }
        int m=0;
        while(m<len1+len2&&num[m]==0){
            m++;
        }
        StringBuilder s=new StringBuilder();
        for(int k=m;k<len1+len2;k++){
            s.append(num[k]);
        }
        return s.toString();
    }
}