# 27. 二进制求和

java

```java
/**
 * 67. 二进制求和.
 * 给定两个二进制字符串，返回他们的和（用二进制表示）。
 *
 * 输入为非空字符串且只包含数字 1 和 0。
 *
 * 示例 1:
 * 输入: a = "11", b = "1"
 * 输出: "100"
 *
 * 示例 2:
 * 输入: a = "1010", b = "1011"
 * 输出: "10101"
 */
public class Solution67 {

    /**
     * 先补空位，然后从后往前计算和
     *
     * @param a
     * @param b
     * @return
     */
    public String addBinary(String a, String b) {
        int max = Math.max(a.length(), b.length());
        int d = 0;
        if (a.length() < max) {
            d = max - a.length();
            for (int i = 0; i < d; i++) a = "0" + a;
        }

        if (b.length() < max) {
            d = max - b.length();
            for (int i = 0; i < d; i++) b = "0" + b;
        }

        String r = "";
        int i=max-1;
        boolean plusOne=false;
        for (; i >= 0; i--) {
            char ch1 = a.charAt(i);
            char ch2 = b.charAt(i);
            int k=ch1-'0'+(ch2-'0');
            if(plusOne) k=k+1;
            if (k>1) {
                k=k-2;
                r = k+ r;
                plusOne=true;
            } else {
                r=k+r;
                plusOne=false;
            }
        }
        if(i<0&&plusOne){
            r="1"+r;
        }
        return r;
    }

    public static void main(String[] args) {
        String a = "1010";
        String b = "1011";
        String r = new Solution67().addBinary(a, b);
        System.out.println(r);//10101
    }
}
```

python

```
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://im-qianuxn.gitbook.io/pytorch/ji-suan-ji/shua-ti/zhi-shang-lei/27-er-jin-zhi-qiu-he.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.