Solution.java

//*
// Target is to take strings representing binary numbers parcels,
// sum them (binary fashion) and return string representing the resulting binary number
// for the reference same code is one line in Clojure, 
// which also works for arbritary number of parcels but it is limited by integer representation:
// <pre>(defn bin-sum [bns] (Integer/toString (apply + (map parse-bin bns)) 2))</pre>
//*
class Solution {
    public String addBinary(String a, String b) {
        int minLength = a.length() < b.length() ? a.length() : b.length();
        int maxLength = a.length() > b.length() ? a.length() : b.length();
        //padding 0s
        StringBuilder pad = new StringBuilder();
        while(pad.length() < (maxLength - minLength)) {
            pad.append('0');
        }
        if(a.length()==minLength) {
            a = pad.append(a).toString();
        } else {
            b = pad.append(b).toString();
        }
        // System.out.println(a);
        // System.out.println(b);
        //real work
        int carry = 0;
        StringBuilder r = new StringBuilder();
        for(int i = maxLength-1; i >=0; i--) {
            // System.out.print(i+"-");
            if(carry==0) {
                if(a.charAt(i)=='0' && b.charAt(i)=='0') {
                    r.append('0');
                } else if(a.charAt(i)=='1' && b.charAt(i)=='0') {
                    r.append('1');
                } else if(a.charAt(i)=='0' && b.charAt(i)=='1') {
                    r.append('1');
                } else if(a.charAt(i)=='1' && b.charAt(i)=='1') {
                    r.append('0');
                    carry = 1;
                }
            } else {
                if(a.charAt(i)=='0' && b.charAt(i)=='0') {
                    r.append('1');
                    carry=0;
                } else if(a.charAt(i)=='1' && b.charAt(i)=='0') {
                    r.append('0');
                } else if(a.charAt(i)=='0' && b.charAt(i)=='1') {
                    r.append('0');
                } else if(a.charAt(i)=='1' && b.charAt(i)=='1') {
                    r.append('1');
                }
            }
        }
        if(carry==1) {
            r.append('1');
        }
        return r.reverse().toString();
    }
}