Add Binary leetcode

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

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

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二进制string加法，可优化代码，不过算法已经接近最优

string addBinary(string a, string b) {
    string ret;
    int lenA = a.size() - 1;
    int lenB = b.size() - 1;
    int cur;
    int add = 0;
    while (lenA >= 0 || lenB >= 0)
    {
        int numA = lenA >= 0 ? a[lenA] - '0' : 0;
        int numB = lenB >= 0 ? b[lenB] - '0' : 0;
        int num = numA + numB + add;
        cur = num & 1;
        add = num >> 1;
        ret.insert(0, 1, char('0' + cur));
        lenA--;
        lenB--;
    }
    if (add == 1)
        ret.insert(0, 1, '1');
    return ret;
}