LeetCode Add Binary

class Solution {
public:
    string addBinary(string a, string b) {
        string res;
        int carry = 0, s = 0;
        int apos = a.length() - 1;
        int bpos = b.length() - 1;
        int astp = 0, bstp = 0;
        // skip leading zero(s)
        for (; astp < apos && a[astp] == '0'; astp++);
        for (; bstp < bpos && b[bstp] == '0'; bstp++);
        // a/b pos validation
        bool apv = apos >= astp, bpv = bpos >= bstp;
        // addition
        while(apv || bpv) {
            if (apv) s += a[apos] - '0';
            if (bpv) s += b[bpos] - '0';
            carry = s > 1;
            s = s & 0x1;
            res.push_back(s ? '1' : '0');
            apos--, bpos--, s = carry;
            apv = apos >= astp, bpv = bpos >= bstp;
        }
        if (carry) res.push_back('1');
        if (res.empty()) {
            res.push_back('0');
        } else {
            reverse(res.begin(), res.end());
        }
        return res;
    }
};

没研究过C++里字符串操作的效率问题，直接使用+进行字符串连接并重新赋值回去不知道额外开销大不大，从提交的结果来看和用push_back的没什么区别，有的甚至更快