Add Binary

 1 class Solution {
 2 public:
 3     string addBinary(string a, string b) {
 4         if( a.empty() ) return b;
 5         if( b.empty() ) return a;
 6         string ans;
 7         reverse(a.begin(), a.end());    //反转后，从低位开始相加
 8         reverse(b.begin(), b.end());
 9         int i=0;
10         int carry = 0;
11         while( i<a.length() && i<b.length() ) { //处理他们共同都有的部分
12             int sum = a[i] - ‘0‘ + b[i] - ‘0‘ + carry;
13             if( sum == 3 ) {
14                 ans.push_back(‘1‘);
15                 carry = 1;
16             } else if( sum == 2 ) {
17                 ans.push_back(‘0‘);
18                 carry = 1;
19             } else if( sum == 1 ) {
20                 ans.push_back(‘1‘);
21                 carry = 0;
22             } else if( sum == 0 ) {
23                 ans.push_back(‘0‘);
24                 carry = 0;
25             }
26             ++i;
27         }
28         string& c = a.length() > b.length() ? a : b;
29         while( i<c.length() ) { //处理一方多余的部分，及进位
30             int sum = c[i] - ‘0‘ + carry;
31             if( sum == 2 ) {
32                 ans.push_back(‘0‘);
33                 carry = 1;
34             } else if( sum == 1 ) {
35                 ans.push_back(‘1‘);
36                 carry = 0;
37             } else if( sum == 0 ) {
38                 ans.push_back(‘0‘);
39                 carry = 0;
40             }
41             ++i;
42         }
43         if( carry ) ans.push_back(‘1‘); //最后若有进位
44         reverse(ans.begin(), ans.end());
45         return ans;
46     }
47 };