Add Binary

从末尾开始相加，本位和进位，得到的本位直接替换掉长串的那一位；如果进位==0且短串已全部遍历完，则跳出循环；

跳出循环后，再判断进位是否>0，大于的补上1。

注意：

1.不能用long，因为字符串的长度方法返回的是int

2.char转化为int，int i = c - '0',ASCII码相减

3. StringBuilder的replace函数，见API

public String addBinary(String a, String b) {        int l1 = a.length()-1;        int l2 = b.length()-1;        int cha = Math.abs(l1-l2);        StringBuilder longStr = new StringBuilder();        StringBuilder shortStr = new StringBuilder();        if(l1 < l2){            longStr = new StringBuilder(b);            shortStr = new StringBuilder(a);        }else{            longStr = new StringBuilder(a);            shortStr = new StringBuilder(b);        }        int jin = 0;        int cur = 0;        int maxLen = longStr.length()-1;        for(int i = maxLen;i>=0;i--){            int lc = longStr.charAt(i)-'0';            int sc = 0;            if(i-cha >= 0){            sc = shortStr.charAt(i-cha)-'0';            }            cur = (lc+sc+jin)%2;            if(lc+sc+jin > 1){                jin = 1;            }else{                jin = 0;            }            longStr.replace(i, i+1, cur+"");            if((i-cha) < 0 && jin == 0)break;        }        if(jin > 0){            longStr.replace(0, 0, "1");        }        return longStr.toString();    }