LeetCode刷题——各位相加和字符串相乘

class Solution {

    public int addDigits(int num) {

        if(num / 10 == 0) return num;

        int tmp = 0;

        for(int i = 10; num > 0; i *= 10) {

            tmp += num % 10;

            num /= 10;

        }

        return addDigits(tmp);

    }

}

class Solution {

    public String multiply(String num1, String num2) {

        int len1=num1.length(),len2=num2.length();

        if(len1<1||len2<1){

            return "0";

        }

        if(num1.equals("0")||num2.equals("0")){

            return "0";

        }

        int[] res=new int[len1+len2];

        for(int i=len1-1;i>=0;--i){

            for(int j=len2-1;j>=0;--j){

                int temp=(num1.charAt(i)-'0')*(num2.charAt(j)-'0')+res[i+j+1];

                res[i+j+1]=temp%10;

                res[i+j]+=temp/10;

            }

        }

        int start=0;

        while(res[start]==0&&start<len1+len2){

            start++;

        }

        StringBuilder ans=new StringBuilder();

        for(int i=start;i<res.length;++i){

            ans.append(res[i]);

        }

        return ans.toString();


    }

}