Leetcode:415. Add Strings

结合着这篇http://www.cnblogs.com/Michael2397/p/8027782.html自己完全实现了，顿时感觉到刷题有套路啊

public class TwoSumbin {
    public String addStrings(String num1, String num2) {
        if(num1==null||num2==null){
            return num1==null?num2:num1;
        }
        StringBuffer string = new StringBuffer();
        int carry=0;
        for (int i = num1.length()-1,j=num2.length()-1; i>=0||j>=0||carry>0; i--,j--) {
            int sum = 0;
            sum +=(i>=0)?num1.charAt(i)-'0':0;
            sum +=(j>=0)?num2.charAt(j)-'0':0;
            sum += carry;
            carry = sum/10;
            sum = sum%10;
            string.append(sum);
        }
    
        return string.reverse().toString();
    }
}