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. 

题目翻译：

给定两个二进制字符串 a 和 b，将它们的和作为二进制字符串返回。

class Solution {
    // 定义方法，将两个二进制字符串相加并返回结果字符串
    public String addBinary(String a, String b) {
        // 创建一个StringBuilder来存储结果字符串
        StringBuilder sb = new StringBuilder();
        // 初始化两个字符串的下标和进位数
        int i = a.length() - 1, j = b.length() - 1, carry = 0;
        // 从末尾开始遍历两个字符串
        while (i >= 0 || j >= 0) {
            int sum = carry;
            // 如果a字符串还有数字，将它加入sum
            if (i >= 0) {
                sum += a.charAt(i) - '0';
                i--;
            }
            // 如果b字符串还有数字，将它加入sum
            if (j >= 0) {
                sum += b.charAt(j) - '0';
                j--;
            }
            // 将sum的个位数加入结果字符串，carry为sum的十位数
            sb.append(sum % 2);
            carry = sum / 2;
        }
        // 如果还有进位，将其加入结果字符串
        if (carry != 0) {
            sb.append(carry);
        }
        // 返回结果字符串，需要先反转再转化为字符串
        return sb.reverse().toString();
    }
}