_67.java

package com.fishercoder.solutions;

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();
        }
    }
}