import java.util.Arrays; public class AddBinary { public static void main(String[] args) { String a = "0"; String b = "1"; AddBinary c = new AddBinary(); System.out.println(c.addBinary(a, b)); } public String addBinary(String a, String b) { if(a == null || b == null || a.length() == 0 || b.length() == 0) return ""; if(a.equals("0") && b.equals("0")) return "0"; char[] a_ = a.toCharArray(); char[] b_ = b.toCharArray(); int min = 0, max = 0; boolean first = false; if(a.length() < b.length()) { min = a.length(); max = b.length(); } else { min = b.length(); max = a.length(); first = true; } char[] ans = new char[max + 1]; int carry = 0; int index = ans.length - 1; for(int i = min - 1; i >= 0; i--) { int sum = Integer.parseInt(a_[i] + "") + Integer.parseInt(b_[i] + "") + carry; // System.out.println(sum + " " + a_[i] + " " + b_[i] + " " + carry); if(sum == 2) { ans[index] = '0'; carry = 1; } else if(sum == 3) { ans[index] = '1'; carry = 1; } else if(sum == 1) { ans[index] = '1'; carry = 0; } else { ans[index] = '0'; carry = 0; } index--; } for(int i = min - 1; i < max; i++) { // System.out.println(a_[i] + " " + b_[i] + " " + carry); if(first) { int sum = Integer.parseInt(a_[i] + "") + carry; if(sum == 2) { ans[index] = '0'; carry = 1; } else if(sum == 3) { ans[index] = '1'; carry = 1; } else if(sum == 1) { ans[index] = '1'; carry = 0; } else { ans[index] = '0'; carry = 0; } index--; } else { int sum = Integer.parseInt(b_[i] + "") + carry; if(sum == 2) { ans[index] = '0'; carry = 1; } else if(sum == 3) { ans[index] = '1'; carry = 1; } else if(sum == 1) { ans[index] = '1'; carry = 0; } else { ans[index] = '0'; carry = 0; } index--; } } if(carry == 1) ans[0] = '1'; // all 0's if(ans[0] == '0') for(int i = 0; i < ans.length; i++) { if(ans[i] == '0') { ans[i] = '\u0000'; } else break; } return new String(ans); } }