[LeetCode] Add Binary

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

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

思路：时间复杂度O(n), 空间复杂度O(1)

class Solution {
public:
    string addBinary(string a, string b) {
        string result;
        if (a.size() == 0 || b.size() == 0) {
            return a.size() == 0 ? b : a;
        }
        
        int carry  = 0;
        int i = a.size() - 1;
        int j = b.size() - 1;
        while (i >= 0 || j >= 0) {
            const int a_val = i >= 0 ? a[i--] - '0': 0;
            const int b_val = j >= 0 ? b[j--] - '0': 0;
            int temp_sum = a_val + b_val + carry;
            result.push_back((temp_sum % 2) + '0');
            carry = temp_sum / 2;
        }
        
        if (carry == 1) result.push_back('1');
        reverse(result.begin(), result.end());
        return result;
    }
};