# LeetCode 67. Add Binary

Given two binary strings `a` and `b`, return *their sum as a binary string*.

**Example 1:**

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

**Example 2:**

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

**Constraints:**

* `1 <= a.length, b.length <= 104`
* `a` and `b` consist only of `'0'` or `'1'` characters.
* Each string does not contain leading zeros except for the zero itself.

## Solution

[English Version in Youtube](https://youtu.be/LIe-KekRmGE)

[中文版解答Youtube Link](https://youtu.be/rZUpzKi0w50)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1LQ4y1Z7MB/)

```
class Solution {
public:
    string addBinary(string a, string b) {
        string result;
        int carry = 0;
        int i1 = a.length() - 1;
        int i2 = b.length() - 1;
        while (i1 >= 0 || i2 >= 0 || carry > 0) {
            int v1 = i1 >= 0 ? a[i1] - '0' : 0;
            int v2 = i2 >= 0 ? b[i2] - '0' : 0;
            int sum = v1 + v2 + carry;
            result.push_back((sum & 1) + '0');
            carry = (sum >> 1) & 1;
            i1--;
            i2--;
        }
        
        reverse(begin(result), end(result));
        return result;
    }
};
```