Add Binary

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

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

Analyse: Using XOR(^) to compute the result of the bit-add and line 22 to reserve the carry bit. Be cautious about the last carry.

 1 class Solution {
 2 public:
 3     string addBinary(string a, string b) {
 4         int carry = 0;
 5         string result;
 6         for(int i = a.length() - 1, j = b.length() - 1; i >= 0 || j >= 0; i--, j--){
 7             int sa, sb;
 8             if(i < 0) {
 9                 sa = 0; 
10                 sb = b[j] - '0';
11             }
12             else if(j < 0){
13                 sb = 0; 
14                 sa = a[i] - '0';
15             }
16             else{
17                 sa = a[i] - '0';
18                 sb = b[j] - '0';
19             }
20             char current = sa ^ sb ^ carry + '0';
21             result = current + result;
22             if((carry + sa + sb) / 2) carry = 1;
23             else carry = 0;
24         }
25         if(carry) result = '1' + result;
26         return result;
27     }
28 };