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