415. Add Strings*

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

1. The length of both num1 and num2 is < 5100.
2. Both num1 and num2 contains only digits 0-9.
3. Both num1 and num2 does not contain any leading zero.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.   

    public String addStrings(String num1, String num2) {        StringBuilder s = new StringBuilder();        int carry = 0;        for(int i= num1.length()-1,  j=num2.length()-1;i>-1||j>-1||carry==1;i--,j--){            int x = i<0?0:num1.charAt(i)-'0';            int y = j<0?0:num2.charAt(j)-'0';            s.append((x+y+carry)%10);            carry =(x+y+carry)/10;        }        return s.reverse().toString();    }

总结：string和int的转换除了使用Integer外还可以对'0'，'a'，‘A’等做减法，记住。