Add Binary - leetcode

Add Binary -- leetcode

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

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

class Solution {
public:
    string addBinary(string a, string b) {
        string ans;
        ans.reserve(max(a.size(), b.size()) + 1);
        
        for (int i=a.size()-1, j=b.size()-1, carry = 0; i>=0 || j>=0 || carry; i--, j--) {
            const int sum = (i>=0 ? a[i]-'0': 0) + (j>=0 ? b[j]-'0' : 0) + carry;
            ans.push_back(sum % 2 + '0');
            carry = sum / 2;
        }
        
        std::reverse(ans.begin(), ans.end());
        return ans;
    }
};