AddBinary.java

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

For example,
a = "11"
b = "1"
Return "100".

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*/

public class AddBinary {
 public String addBinary(String a, String b) {
 int i = a.length() - 1, j = b.length() - 1;
 StringBuilder sb = new StringBuilder();
 int carry = 0;
 while (i >= 0 || j >= 0) {
 int sum = carry;
 if (i >= 0) sum += a.charAt(i--) - '0';
 if (j >= 0) sum += b.charAt(j--) - '0';
 sb.append(sum % 2);
 carry = sum / 2;
 }
 if (carry > 0) sb.append("1");
 return sb.reverse().toString();
 }
}