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"
解：很巧妙。
class Solution {
public String addBinary(String a, String b) {
StringBuilder result = new StringBuilder();
int pointerA = a.length() - 1;
int pointerB = b.length() - 1;
//进位标志
int carry = 0;
while (pointerA >= 0 || pointerB >= 0) {
int sum = carry;
if (pointerA >= 0) {
sum += (a.charAt(pointerA) - '0');
pointerA--;
}
if (pointerB >= 0) {
sum += (b.charAt(pointerB) - '0');
pointerB--;
}
result.append(sum % 2);
carry = sum / 2;
}
if (carry != 0) {
result.append('1');
}
return result.reverse().toString();
}
}
