67. Add Binary

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"

注意进位。
solution1：

class Solution {
    public String addBinary(String a, String b) {
        int carry = 0;
        int aDigit = 0;
        int bDigit = 0;
        StringBuilder result = new StringBuilder();
        
        for(int i = a.length() - 1, j = b.length() - 1; i >= 0 || j >=0; i--, j--) {
            aDigit = (i > -1) ? a.charAt(i) - '0' : 0;
            bDigit = (j > -1) ? b.charAt(j) - '0' : 0;
            
            result.append((carry + aDigit + bDigit) % 2);                   // Appends String rep of int argument
            carry = (carry + aDigit + bDigit) / 2;
        }
        
        if(carry > 0) {
            result.append(carry);
        }
        
        return result.reverse().toString();
        
    }
}

　　It's just like in Add TwoNumbers, we add from the last digits of two numbers and keep update carry.

Remember to care the last carry digit.

Then reverse and return.