Add Binary - String - Easy - LeetCode

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

The input strings are both non-empty and contain only characters 1 or 0.

Example 1:

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

Output: "100"

Example 2:

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

Output: "10101"

Constraints:

Each string consists only of '0' or '1' characters.

1 <= a.length, b.length <= 10^4

Each string is either "0" or doesn't contain any leading zero.

Solution:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace LeetCode.AskGif.Easy.String
{
 public class AddBinarySoln
 {
 public string AddBinary(string a, string b)
 {
 int len1 = a.Length;
 int len2 = b.Length;
 char carry = '0';
 var diffStr = new StringBuilder();
 int diff;
 if (len1 > len2)
 {
 diff = len1 - len2;
 while (diff-- > 0)
 {
 diffStr.Append("0");
 }
 b = diffStr.ToString() + b;
 }
 else
 {
 diff = len2 - len1;
 while (diff-- > 0)
 {
 diffStr.Append("0");
 }
 a = diffStr.ToString() + a;
 }

 var res = new StringBuilder();
 for (int i = a.Length-1; i >= 0 ; i--)
 {
 string sum = Addition(a[i], b[i], carry); 
 if(sum.Length == 2)
 {
 res.Append(sum[1]);
 carry = sum[0];
 }
 else
 {
 res.Append(sum[0]);
 carry = '0';
 }
 }

 if(carry == '1')
 {
 res.Append(carry);
 }

 return new string (res.ToString().Reverse().ToArray());
 }

 private string Addition(char v1, char v2, char carry)
 {
 if(carry == '0')
 {
 if (v1 == '1' && v2 == '1')
 {
 return "10";
 }
 else if (v1 == '0' && v2 == '0')
 {
 return "0";
 }
 }
 else
 {
 if (v1 == '1' && v2 == '1')
 {
 return "11";
 }
 else if (v1 == '0' && v2 == '0')
 {
 return "1";
 }
 else
 {
 return "10";
 }
 } 
 return "1"; 
 }
 }
}

Time Complexity: O(n)

Space Complexity: O(1)

Unit Tests:

using LeetCode.AskGif.Easy.String;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;
using System.Collections.Generic;
using System.Text;

namespace CodingUnitTest.Easy.String
{
 [TestClass]
 public class AddBinarySolnTests
 {
 [TestMethod]
 public void AddBinarySoln_First()
 {
 var a = "11";
 var b = "1";
 var output = "100";
 var res = new AddBinarySoln().AddBinary(a, b);

 Assert.AreEqual(res, output);
 }

 [TestMethod]
 public void AddBinarySoln_Second()
 {
 var a = "1010";
 var b = "1011";
 var output = "10101";
 var res = new AddBinarySoln().AddBinary(a, b);

 Assert.AreEqual(res, output);
 }
 }
}