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 };