C# Coding Interview Questions – Strings and Arrays

1. How do you reverse a string without using built-in reverse methods?

Explanation: Reverse a string by reading its characters from the last position to the first position.

Example:

string input = "Hello";
string reversed = "";

for (int i = input.Length - 1; i >= 0; i--)
{
    reversed += input[i];
}

Console.WriteLine(reversed);

Output:

olleH

How it works:

Input:   H e l l o
Index:   0 1 2 3 4

Read:    4 -> 3 -> 2 -> 1 -> 0

Output:  o l l e H

Key Points:

• Length - 1 identifies the last character.

• The loop runs backward.

• Time complexity: O(n²) with repeated string concatenation in this implementation.

• For large strings, use char[] or StringBuilder for O(n) performance.

2. How do you find duplicate elements in an array? 🔴

Explanation: Duplicate elements are values that appear more than once in an array.

Example:

int[] numbers = { 10, 20, 30, 20, 40, 10 };

HashSet<int> seen = new HashSet<int>();
HashSet<int> duplicates = new HashSet<int>();

foreach (int number in numbers)
{
    if (!seen.Add(number))
    {
        duplicates.Add(number);
    }
}

foreach (int number in duplicates)
{
    Console.WriteLine(number);
}

Output:

20
10

How it works:

• seen stores numbers already encountered.

• seen.Add() returns false when a number already exists.

• Duplicate numbers are added to duplicates.

• HashSet prevents duplicate results.

Key Points:

• HashSet<T> stores unique elements.

• Add() returns true for a new element.

• Average time complexity: O(n).

• Additional space complexity: O(n).

3. How do you find the second-highest number in an array? 🔴

Explanation: Find the second-largest distinct value without sorting the array.

Example:

int[] numbers = { 10, 50, 30, 40, 20 };

int? highest = null;
int? secondHighest = null;

foreach (int number in numbers)
{
    if (highest == null || number > highest)
    {
        secondHighest = highest;
        highest = number;
    }
    else if (number < highest &&
             (secondHighest == null || number > secondHighest))
    {
        secondHighest = number;
    }
}

Console.WriteLine(secondHighest?.ToString() ?? "Not found");

Output:

40

How it works:

Array: 10, 50, 30, 40, 20

Highest       = 50
SecondHighest = 40

Result = 40

Key Points:

• No sorting is required.

• Uses a single loop.

• Time complexity: O(n).

• Space complexity: O(1).

• Duplicate maximum values are ignored.

• Returns "Not found" if there are fewer than two distinct values.

4. How do you remove duplicate characters from a string? 🔴

Explanation: Remove repeated characters while preserving the order of their first appearance.

Example:

string input = "programming";
string result = "";

HashSet<char> seen = new HashSet<char>();

foreach (char ch in input)
{
    if (seen.Add(ch))
    {
        result += ch;
    }
}

Console.WriteLine(result);

Output:

progamin

How it works:

Input:  p r o g r a m m i n g

Remove repeated:
        r, m, g

Output: p r o g a m i n

Key Points:

• HashSet<char> identifies repeated characters.

• Maintains the original character order.

• Character comparison is case-sensitive (A and a are different).

• For large strings, use StringBuilder instead of repeated concatenation.

5. How do you check whether a string is a palindrome? 🔴

Explanation: A palindrome is a string that reads the same forward and backward.

Examples: madam, level, radar.

Example:

string input = "madam";

int left = 0;
int right = input.Length - 1;

bool isPalindrome = true;

while (left < right)
{
    if (input[left] != input[right])
    {
        isPalindrome = false;
        break;
    }

    left++;
    right--;
}

Console.WriteLine(isPalindrome);

Output:

True

How it works:

Input: m a d a m
       0 1 2 3 4

Step 1: m == m  -> True
Step 2: a == a  -> True

Result: Palindrome

Key Points:

• Uses the two-pointer technique.

• Compares characters from both ends.

• Stops immediately when characters differ.

• Time complexity: O(n).

• Space complexity: O(1).

• This example is case-sensitive and includes spaces in comparisons.

Quick Interview Revision

Interview Tip: For coding interviews, explain the approach first, write the code, and finally mention time complexity and space complexity.