Add Binary

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

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

题意：将两个代表二进制数的字符串相加得到一个二进制结果

代码：

public class Solution {
    public String addBinary(String a, String b) {
        int length1 = a.length(), length2 = b.length();
		if(length1 < length2)
			return addBinary(b, a);
		String pre = "",result = "", rr = "";
		for(int i = 1; i <= length1 - length2; i++)
			pre += "0";
		b = pre + b;
        int i = length1 - 1,tmp = 0; 
        while(i >= 0)
        {
        	int aa = a.charAt(i) - '0';
        	int bb = b.charAt(i) - '0';
        	result += (aa + bb + tmp) % 2;
        	tmp = (aa + bb + tmp) / 2;
        	i--;
        }
        if(tmp == 1)
        	result += tmp;
        int length = result.length();
        for(i = length - 1; i >= 0; i--)
        	rr += result.charAt(i);
        return rr;
    }
}