67. Add Binary

Given two binary strings, return their sum (also a binary string).

For example,

a = "11"

b = "1"

Return "100".

public class Solution {
    public String addBinary(String a, String b) {
        if(a == null && b == null ) return null;
        if(a == null || b == null) return a == null ? b :a;
        StringBuilder sb = new StringBuilder();

        int p = a.length() - 1;
        int q = b.length() - 1;
        int carry = 0;

        while(p >=0 || q >=0){
            int i1 = p >= 0 ? a.charAt(p--) - '0' : 0;
            int i2 = q >= 0 ? b.charAt(q--) - '0' : 0;

            sb.append((i1+i2+carry)%2);
            carry = (i1+i2+carry)/2;
        }

        if(carry != 0){
            sb.append(carry);
        }

        return sb.reverse().toString();

    }
}

simpler code

public class Solution {
    public String addBinary(String a, String b) {
        int carry = 0;
        int al = a.length()-1, bl = b.length()-1;
        StringBuilder sb = new StringBuilder();
        while(al >=0 || bl>=0 || carry > 0){
            int x = al >= 0 ? a.charAt(al--) - '0' : 0;
            int y = bl >= 0 ? b.charAt(bl--) - '0' : 0;

            sb.append((x+y+carry)%2);
            carry = (x+y+carry)/2;
        }

        return sb.reverse().toString();
    }
}