Multiply Strings

题中要求num1和num2最大可达到110位，因此不能进行整体计算，需要一位一位进行计算，所以按照小学学习的乘法算法用代码实现即可。
两个数相乘，其结果最长不会超过两个数长度之和，因此可提前拟定好结果，每次运算相加即可。代码如下

public class multiply {
    public static String multiply(String num1, String num2) {
        char[] c1 = num1.toCharArray();
        char[] c2 = num2.toCharArray();
        char[] cr = new char[c1.length+c2.length];
        for(int i=0;i<cr.length;i++){
            cr[i]='0';
        }
        for(int i=c1.length-1;i>=0;i--){
            for(int j=c2.length-1;j>=0;j--){
                int temp = toInt(c1[i]) * toInt(c2[j]);
                if(temp==0)continue;
                int location = cr.length-c1.length-c2.length+i+j+1;
                int pre=0;
                while(temp!=0 || pre!=0){
                    int remainder = temp%10 + pre;
                    temp = temp/10;
                    int tempRes = toInt(cr[location])+remainder;
                    if(tempRes<10){
                        cr[location] = toChar(tempRes);
                        pre=0;
                    }else {
                        cr[location] = toChar(tempRes%10);
                        pre=tempRes/10;
                    }
                    location--;
                }

            }
        }
        StringBuilder res = new StringBuilder();
        boolean f = false;
        for(int i=0;i<cr.length;i++){
            if(cr[i]!='0' || f){
                res.append(cr[i]);
                f=true;
            }
        }
        if(res.length()==0)return "0";
        return res.toString();
    }

    private static int toInt(char c){
        return c-48;
    }

    private static char toChar(int c){
        return (char)(c+48);
    }

    public static void main(String[] args) {
        System.out.println(multiply("123","0"));
    }
}