Add Binary Leetcode java

题目：

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

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

题解：

二进制加法都是从最低位（从右加到左）。所以对两个字符串要从最后一位开始加，如果遇见长度不一的情况，就把短的字符串高位补0.

每轮计算要加上进位，最后跳出循环后要坚持进位是否为1，以便更新结果。

代码如下（from discussion）：

 1 public String addBinary(String a, String b) {
 2     int m = a.length();
 3     int n = b.length();
 4     int carry = 0;
 5     String res = "";
 6     // the final length of the result depends on the bigger length between a and b, 
 7     // (also the value of carry, if carry = 1, add "1" at the head of result, otherwise)
 8     int maxLen = Math.max(m, n);
 9     for (int i = 0; i < maxLen; i++) {
10         // start from last char of a and b
11         // notice that left side is int and right side is char
12         // so we need to  minus the decimal value of '0'
13         int p=0,q=0;
14         if(i<m)
15             p = a.charAt(m-1-i) - '0';
16         else
17             p = 0;
18         
19         if(i<n)
20             q = b.charAt(n-1-i)-'0';
21         else
22             q = 0;
23             
24         int tmp = p + q + carry;
25         carry = tmp / 2;
26         res += tmp % 2;
27     }
28     return (carry == 0) ? res : "1" + res;
29     }