Add Binary(67 Easy)

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"

Return "100".

public String addBinary(String a, String b) {
    int i = a.length() - 1, j = b.length() - 1, carry = 0;
    StringBuilder sb = new StringBuilder();
    while(i >=0 || j >=0){
        int m = i >= 0 ? a.charAt(i) - '0' : 0;
        int n = j >= 0 ? b.charAt(j) - '0' : 0;
        int sum = m + n + carry;
        carry = sum / 2;
        sb.insert(0, String.valueOf(sum % 2));
        i--;
        j--;
    }
    if(carry != 0) sb.insert(0, '1');
    return sb.toString();
}