415. Add Strings

这题其实写起来有点烦的 注意index不要超，使用i作count会比较方便。

class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();
        int len1 = num1.length();
        int len2 = num2.length();
        int longer = len1>len2?len1:len2;
        int shorter = longer==len1? len2:len1;
        int carry = 0;
        // use i for count not index 
        for(int i = 0;i<shorter;i++)
        {
            int n1 = (int)(num1.charAt(len1-1-i)-'0');
            int n2 = (int)(num2.charAt(len2-1-i)-'0');
            int num = (n1+n2+carry)%10;
            carry=(n1+n2+carry)/10;
            sb.insert(0,num);
        }
        for(int i = 0;i<longer-shorter;i++)
        {
            if(len1==longer)
            {
                 int n1 = (int)(num1.charAt(len1-shorter-1-i)-'0');
                 int num = (n1+carry)%10;
                 carry=(n1+carry)/10;
                System.out.println(num);
                 sb.insert(0,num);
            }
            else
            {
              int n2 = (int)(num2.charAt(len2-shorter-1-i)-'0');
              int num = (n2+carry)%10;
              carry=(n2+carry)/10;
              sb.insert(0,num);
            }
        }
        if(carry==1)
            sb.insert(0,1);
        return sb.toString();
    }
}