Add Strings

Given two non-negative integers, num1 and num2 represented as a string, return the sum of num1 and num2.

Note:

The length of both num1 and num2 is < 5100.

Both num1 and num2 contains only digits 0-9.

Both num1 and num2 does not contain any leading zero.

You must not use any built-in BigInteger library or convert the inputs to integer directly.

Solution:

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

namespace LeetCode.AskGif.Easy.String
{
 public class AddStringsSoln
 {
 public string AddStrings(string num1, string num2)
 {
 int len1 = num1.Length;
 int len2 = num2.Length;
 int diff = 0;
 var diffStr = new StringBuilder();
 if (len1 > len2)
 {
 diff = len1 - len2;
 for (int i = 0; i < diff; i++)
 {
 diffStr.Append("0");
 }
 num2 = diffStr.ToString() + num2;
 }
 else
 {
 diff = len2 - len1;
 for (int i = 0; i < diff; i++)
 {
 diffStr.Append("0");
 }
 num1 = diffStr.ToString() + num1;
 }

 int carry = 0;
 int val = 0;
 var res = new StringBuilder();
 for (int i = num1.Length-1; i >= 0; i--)
 {
 int n = AsciiToNum(num1[i]) + AsciiToNum(num2[i]) + carry;
 if (n > 9)
 {
 val = n % 10;
 res.Append(val.ToString());
 carry = n / 10;
 }
 else
 {
 res.Append(n.ToString());
 carry = 0;
 }
 }
 if (carry > 0)
 {
 res.Append(carry);
 }

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

 private int AsciiToNum(char ch)
 {
 return ch - 48;
 }

 }
}

Time Complexity: O(n)

Space Complexity: O(n)

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 AddStringsSolnTests
 {
 [TestMethod]
 public void AddStringsSoln_First()
 {
 var num1 = "12";
 var num2 = "18";
 var output = "30";
 var res = new AddStringsSoln().AddStrings(num1, num2);

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