67. 二进制字符串相加 Add Binary

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

For example,
a = "11"
b = "1"
Return "100".

public class Solution {
    public string AddBinary(string a, string b) {
		int length = Math.Max(a.Length, b.Length);
		a = a.PadLeft(length, '0');
		b = b.PadLeft(length, '0');
		string sum = "";
		int carry = 0;
		for (int i = length - 1; i >= 0; i--) {
			int s = (a[i]-48) + (b[i]-48);
			if (carry > 0) {
				s++;
				carry--;
			}
			if (s > 1) {
				carry++;
			}
			s = s % 2;
			sum = s.ToString() + sum;
		}
		if (carry > 0) {
			sum = "1" + sum;
		}
		return sum;
    }
}

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