67. Add Binary

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

Add like on paper: walk both strings from the right, sum the two digits and the carry, write sum % 2 and carry sum >> 1.

1function addBinary(a: string, b: string): string {
2  let i = a.length - 1, j = b.length - 1, carry = 0, out = "";
3  while (i >= 0 || j >= 0 || carry) {
4    const sum = (i >= 0 ? +a[i--] : 0) + (j >= 0 ? +b[j--] : 0) + carry;
5    out = (sum % 2) + out;
6    carry = sum >> 1;
7  }
8  return out;
9}

carry = 0

a

b

out

empty

Start at the rightmost digit of each string.

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