LeetCode 67. Add Binary

Question

Given two binary strings a and b, return their sum as a binary string.

Example 1:

Input: a = "11", b = "1"
Output: "100"

Example 2:

Input: a = "1010", b = "1011"
Output: "10101"

Constraints:

• 1 <= a.length, b.length <= 104
• a and b consist only of '0' or '1' characters.
• Each string does not contain leading zeros except for the zero itself.

Source: https://leetcode.com/problems/add-binary/

Solution

public String addBinary(String a, String b) {
    if (a == null && b == null) {
        return "";
    } else if (a == null) {
        return b;
    } else if (b == null) {
        return a;
    }
    int lenA = a.length();
    int lenB = b.length();
    if (lenA == 0 && lenB == 0) {
        return "";
    } else if (lenA == 0) {
        return b;
    } else if (lenB == 0) {
        return a;
    }

    StringBuilder sb = new StringBuilder();
    int ia = lenA - 1, ib = lenB - 1;
    int carry = 0;
    while (ia >= 0 && ib >= 0) {
        int bitA = a.charAt(ia) - '0';
        int bitB = b.charAt(ib) - '0';
        int bitC = (bitA + bitB + carry) % 2;
        carry = (bitA + bitB + carry) / 2;
        sb.append(bitC);
        ia--;
        ib--;
    }
    // these two loops handle remaining bits
    // only one loop will be entered
    while (ia >= 0) {
        int bitA = a.charAt(ia) - '0';
        int bitC = (bitA + carry) % 2;
        carry = (bitA + carry) / 2;
        sb.append(bitC);
        ia--;
    }
    while (ib >= 0) {
        int bitB = b.charAt(ib) - '0';
        int bitC = (bitB + carry) % 2;
        carry = (bitB + carry) / 2;
        sb.append(bitC);
        ib--;
    }
    // if carry is the most significant bit
    if (carry == 1) {
        sb.append(carry);
    }

    sb.reverse();
    return sb.toString();
}