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

For example,

a = "11"

b = "1"

Return "100".

详细一位一位地加即可了，考虑进位的问题。还有最后记得把生成的string反过来再返回，由于我们是从最低位開始加的。

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;

        StringBuilder res = new StringBuilder();

        int i = a.length() - 1;

        int j = b.length() - 1;

        int digit;

        int carry = 0;

        while(i >= 0 && j >= 0){

            digit = (int)(a.charAt(i) - '0' + b.charAt(j) - '0' + carry);

            carry = digit / 2;

            digit %= 2;

            res.append(digit);

            i--;

            j--;

        }

        while(i >= 0){

            digit = (int)(a.charAt(i) - '0' + carry);

            carry = digit / 2;

            digit %= 2;

            res.append(digit);

            i--;

        }

        while(j >= 0){

            digit = (int)(b.charAt(j) - '0' + carry);

            carry = digit / 2;

            digit %= 2;

            res.append(digit);

            j--;

        }

        //don't forget to add the final carry(if exists)

        if(carry > 0){

            res.append(carry);

        }

        return res.reverse().toString();

    }

}