415. Add Strings

没什么限定的话，先翻转，在一位一位加，记得进位就行了。。

public class Solution 
{
    public String addStrings(String num1, String num2) 
    {
         
        StringBuilder sb = new StringBuilder(num1);
        num1 = sb.reverse().toString();
        sb = new StringBuilder(num2);
        num2 = sb.reverse().toString();
        
        if(num1.length() > num2.length())
        {
            String temp = num1;
            num1 = num2;
            num2 = temp;
        }
        String res = new String();
        int carry = 0;
        for(int i = 0; i < num1.length();i++)
        {
            int val = num1.charAt(i)+num2.charAt(i)- '0'-'0'+ carry;
            if(val > 9) carry = 1;
            else carry = 0;
            val %= 10;
            res+=Integer.toString(val);
        }
        
        for(int i = num1.length(); i < num2.length();i++)
        {
            int val = num2.charAt(i) - '0' + carry;
            if(val > 9) carry = 1;
            else carry = 0;
            val %= 10;
            res+= Integer.toString(val);
        }
        if(carry == 1) res += 1;
        
        sb = new StringBuilder(res);
        return sb.reverse().toString();
    }
}