Two Pointer Technique on Arrays in DSA C# - Build from Scratch

Two Pointer Technique on Arrays

📖 Scenario: You are working with a list of numbers representing daily temperatures. You want to find pairs of days where the temperature difference is exactly a certain value.

🎯 Goal: Build a program that uses the two pointer technique to find all pairs of numbers in a sorted array whose difference equals a given target.

📋 What You'll Learn

Create an integer array called temperatures with the exact values: 2, 5, 9, 12, 15, 18

Create an integer variable called targetDifference and set it to 3

Use two integer pointers called left and right to find all pairs where the difference between temperatures[right] and temperatures[left] is equal to targetDifference

Print each pair found in the format: (leftValue, rightValue)

💡 Why This Matters

🌍 Real World

Finding pairs of values with a specific difference is useful in tasks like analyzing temperature changes, stock price differences, or time intervals.

💼 Career

Two pointer technique is a common pattern in coding interviews and real-world software development for efficient array processing.

1

Create the temperatures array

Create an integer array called temperatures with these exact values: 2, 5, 9, 12, 15, 18

DSA C#

# Create the temperatures array with the given values
// Your code here

Hint

Use int[] temperatures = {2, 5, 9, 12, 15, 18}; to create the array.

2

Set the target difference

Create an integer variable called targetDifference and set it to 3

DSA C#

int[] temperatures = {2, 5, 9, 12, 15, 18};
// Create targetDifference variable and set it to 3
// Your code here

Hint

Use int targetDifference = 3; to create the variable.

3

Use two pointers to find pairs with the target difference

Create two integer variables called left and right and set them to 0 and 1 respectively. Use a while loop to move the pointers through the temperatures array and find all pairs where temperatures[right] - temperatures[left] == targetDifference. Inside the loop, if the difference is less than targetDifference, move right forward by 1. If the difference is greater than targetDifference, move left forward by 1. If the difference equals targetDifference, print the pair and move both pointers forward by 1.

DSA C#

int[] temperatures = {2, 5, 9, 12, 15, 18};
int targetDifference = 3;
// Use two pointers left and right to find pairs with difference equal to targetDifference
// Your code here

Hint

Use a while loop with left and right pointers and compare the difference to targetDifference.

4

Print all pairs found

Run the program to print all pairs of numbers from temperatures whose difference equals targetDifference. The output should show each pair in the format (leftValue, rightValue) on its own line.

DSA C#

int[] temperatures = {2, 5, 9, 12, 15, 18};
int targetDifference = 3;

int left = 0;
int right = 1;

while (right < temperatures.Length && left < temperatures.Length)
{
    int diff = temperatures[right] - temperatures[left];
    if (diff == targetDifference)
    {
        // Print the pair
        // Your code here
        left++;
        right++;
    }
    else if (diff < targetDifference)
    {
        right++;
    }
    else
    {
        left++;
        if (left == right) right++;
    }
}

Hint

Make sure to print each pair exactly as (leftValue, rightValue) using System.Console.WriteLine.

Practice

1. What is the main idea behind the Two Pointer Technique when used on arrays?

easy

A. Use two indices to traverse the array from different ends or positions simultaneously

B. Use one pointer to traverse and another to store values

C. Sort the array first and then use binary search

D. Use recursion to split the array into two parts

Solution

1. Step 1: Understand the two pointer approach

   The technique uses two pointers (indices) to scan the array from different positions, often from start and end. This helps solve problems efficiently by reducing the number of iterations compared to nested loops.

2. Final Answer:

   Use two indices to traverse the array from different ends or positions simultaneously -> Option A

3. Quick Check:

   Two pointers move from different ends = D [OK]

Hint: Two pointers move from start and end to meet in middle [OK]

Common Mistakes:

• Thinking it always requires sorting first
• Using only one pointer instead of two
• Confusing with recursion or binary search

2. Which of the following is the correct syntax to initialize two pointers left and right for an array arr in C#?

easy

A. int left = -1, right = arr.Length + 1;

B. int left = 1, right = arr.Length;

C. int left = arr.Length, right = 0;

D. int left = 0, right = arr.Length - 1;

Solution

1. Step 1: Understand array indexing and initialize pointers correctly

   Arrays start at index 0 and end at index Length - 1. Left pointer starts at 0 (first element), right pointer at Length - 1 (last element).

2. Final Answer:

   int left = 0, right = arr.Length - 1; -> Option D

3. Quick Check:

   Start at 0 and end at Length-1 = A [OK]

Hint: Start left at 0, right at length minus one [OK]

Common Mistakes:

• Starting right pointer at arr.Length (out of bounds)
• Starting left pointer at 1 instead of 0
• Using negative indices

3. What will be the output of the following C# code?

int[] arr = {1, 2, 3, 4, 5, 6};
int left = 0, right = arr.Length - 1;
int sum = 0;
while (left < right) {
    sum += arr[left] + arr[right];
    left++;
    right--;
}
Console.WriteLine(sum);

medium

A. 21

B. 18

C. 20

D. 15

Solution

1. Step 1: Trace the loop iterations

   Pairs added: (1+6)=7, (2+5)=7, (3+4)=7; sum = 7+7+7 = 21. Loop runs while left < right, so stops after these pairs.

2. Final Answer:

   21 -> Option A

3. Quick Check:

   Sum of pairs from ends = 21 [OK]

Hint: Sum pairs from ends until pointers meet [OK]

Common Mistakes:

• Adding elements twice
• Loop running when left equals right
• Incorrect sum calculation

4. Identify the error in the following C# code using two pointers to find if array arr contains two numbers that sum to target.

int left = 0, right = arr.Length - 1;
while (left <= right) {
    int sum = arr[left] + arr[right];
    if (sum == target) return true;
    else if (sum < target) left++;
    else right--;
}
return false;

medium

A. The sum calculation is incorrect

B. The pointers should both start at 0

C. The loop condition should be left < right instead of left <= right

D. The return statements are missing a value

Solution

1. Step 1: Analyze loop condition

   Using left <= right allows pointers to be equal, causing sum of same element twice. Change to left < right to avoid using the same element twice.

2. Final Answer:

   The loop condition should be left < right instead of left <= right -> Option C

3. Quick Check:

   Pointers must not cross or equal for two distinct elements [OK]

Hint: Use left < right to avoid same element twice [OK]

Common Mistakes:

• Allowing pointers to be equal causing wrong sums
• Starting both pointers at 0
• Misplacing return statements

5. Given a sorted array arr and a number k, how can you use the two pointer technique to find if there exists a pair with sum equal to k? Choose the correct approach.

hard

A. Initialize both pointers at start; move both right until sum equals k

B. Initialize two pointers at start and end; move left pointer right if sum < k, else move right pointer left

C. Initialize two pointers at middle; move both outward until sum equals k

D. Sort the array again and use binary search for each element

Solution

1. Step 1: Set pointers and move based on sum

   Left pointer at 0, right pointer at last index. If sum < k, move left right to increase sum; if sum > k, move right left to decrease sum.

2. Final Answer:

   Initialize two pointers at start and end; move left pointer right if sum < k, else move right pointer left -> Option B

3. Quick Check:

   Two pointers adjust to reach target sum efficiently [OK]

Hint: Move pointers inward based on sum comparison [OK]

Common Mistakes:

• Starting both pointers at same side
• Moving pointers in wrong direction
• Sorting array again unnecessarily