Leetcode: Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

 我自己的做法：用StringBuffer, a和b都从末尾扫描到开头，而StringBuffer则是每次存在开头。循环则是只要aindex>=0 or bindext>=0 or carry>0就继续。加法则是分成3部分，依次判断aindex, bindex, carry是否满足加的条件，满足就加。这样写结构非常清晰。可以作为这类题的解题模板

public class Solution {
    public String addBinary(String a, String b) {
        if (a==null || a.length()==0) return b;
        if (b==null || b.length()==0) return a;
        StringBuffer res = new StringBuffer();
        int aindex = a.length() - 1;
        int bindex = b.length() - 1;
        int digit = 0;
        int carry = 0;
        while (aindex>=0 || bindex>=0 || carry>0) {
            int result = 0;
            if (aindex >= 0) {
                result += (int)(a.charAt(aindex) - '0');
                aindex--;
            }
            if (bindex >= 0) {
                result += (int)(b.charAt(bindex) - '0');
                bindex--;
            }
            if (carry > 0) {
                result += carry;
            }
            digit = result % 2;
            carry = result / 2;
            res.insert(0, digit);
        }
        return res.toString();
    }
}

 别人的代码，用StringBuffer，差不多

public String addBinary(String a, String b) {
    if(a==null || a.length()==0)
        return b;
    if(b==null || b.length()==0)
        return a;
    int i=a.length()-1;
    int j=b.length()-1;
    int carry = 0;
    StringBuilder res = new StringBuilder();
    while(i>=0&&j>=0)
    {
        int digit = (int)(a.charAt(i)-'0'+b.charAt(j)-'0')+carry;
        carry = digit/2;
        digit %= 2;
        res.append(digit);
        i--;
        j--;
    }
    while(i>=0)
    {
        int digit = (int)(a.charAt(i)-'0')+carry;
        carry = digit/2;
        digit %= 2;
        res.append(digit);
        i--;
    }
    while(j>=0)
    {
        int digit = (int)(b.charAt(j)-'0')+carry;
        carry = digit/2;
        digit %= 2;
        res.append(digit);
        j--;
    }      
    if(carry>0)
    {
        res.append(carry);
    }
    return res.reverse().toString();
}