067. Add Binary

At a Glance

• Topic: Bit Manipulation
• Pattern: Math / Simulation
• Difficulty: Easy
• LeetCode: 067

Problem Statement

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 &#39;0&#39; or &#39;1&#39; characters.
Each string does not contain leading zeros except for the zero itself.

Approach & Solution Steps

Iterate through the strings from right to left, simulating binary addition. Keep track of the carry and append the sum modulo 2 to the result string.

Optimal JS Solution

function addBinary(a, b) {
  let result = "";
  let carry = 0;
  let i = a.length - 1, j = b.length - 1;
  while (i >= 0 || j >= 0 || carry > 0) {
    const sum = (i >= 0 ? Number(a[i--]) : 0) + (j >= 0 ? Number(b[j--]) : 0) + carry;
    result = (sum % 2) + result;
    carry = Math.floor(sum / 2);
  }
  return result;
}

Edge Cases & Pitfalls

• Always consider empty or null inputs.
• Watch out for off-by-one index errors.