Leetcode43字符串相乘

代码：

class Solution {
    public String multiply(String num1, String num2) {
        if(num1.equals("0")||num2.equals("0"))return "0";
        int i,j;
        String res = "0";
        for(i = num2.length()-1;i>=0;i--){
            StringBuilder curr = new StringBuilder();
            int f = 0;
            for(int k = num2.length()-1;k>i;k--){
                curr.append(0);
            }
            int y = num2.charAt(i) - '0';
            for(j = num1.length()-1;j>=0;j--){
                int x = num1.charAt(j) - '0';
                int product = x * y + f;
                curr.append(product%10);
                f = product /10;
            }
            if(f!=0)curr.append(f);
            res = addStrings(res,curr.reverse().toString());
        }
        return res;
    }
    public String addStrings(String num1, String num2) {
        StringBuilder res = new StringBuilder();
        int f = 0;
        int i,j;
        for(i=num1.length()-1,j=num2.length()-1;i>=0||j>=0;i--,j--){
            int x = i>=0? num1.charAt(i)-'0' : 0;
            int y = j>=0? num2.charAt(j)-'0' : 0;            
            int curNum = x + y + f;
            if(curNum<=9){
                f = 0;
                res.append(curNum);
            }else{
                f = 1;
                res.append(curNum - 10);               
            }
        }
        if(f==1) res.append(1);
        res.reverse();
        return res.toString();
    }
}