67. Add Binary（二进制求和）

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

The input strings are both non-empty and contains only characters 1 or 0.

Example 1:

Input: a = "11", b = "1"
Output: "100"

Example 2:

Input: a = "1010", b = "1011"
Output: "10101"

class Solution {
public:
    string addBinary(string a, string b) {
        int carry = 0;
        int a_s = a.size()-1, b_s = b.size()-1;
        string res = "";
        while(a_s>=0||b_s>=0) {
            int a_int = a_s >=0 ?a[a_s]-'0':0;
            int b_int = b_s >=0 ?b[b_s]-'0':0;
            int cur = a_int + b_int + carry;
            carry = cur>=2?1:0;
            string cur_res = cur%2==1?"1":"0";
            res.append(cur_res);
            a_s--;b_s--;
        }
        if (carry) res.append("1");
        reverse(res.begin(), res.end());
        return res;
    }
};