_67.java

package com.fishercoder.solutions;

/**
 * 67. Add Binary
 *
 * Given two binary strings, return their sum (also a binary string).
 * For example,
 * a = "11"
 * b = "1"
 * Return "100".
 */

public class _67 {
  public static class Solution1 {
    /**
     * credit: https://discuss.leetcode.com/topic/13698/short-ac-solution-in-java-with-explanation
     * 1. use StringBuilder.reverse() function! Nice!
     * 2. if a numeric number is represented/stored in String, how to get its value: use Character.getNumericValue(s.charAt(i))
     * 3. directly adding/subtracting chars will end up working with their ASCII numbers, e.g. chars[0] = 'a', chars[1] = 'b', then chars[0] + chars[1] will become 195.
     */
    public String addBinary(String a, String b) {
      int carry = 0;
      int i = a.length() - 1;
      int j = b.length() - 1;
      StringBuilder sb = new StringBuilder();
      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(carry);
      }
      return sb.reverse().toString();
    }
  }
}