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

Two Pointer Technique on Arrays

📖 Scenario: You are working with a sorted list of numbers representing daily temperatures in Celsius. You want to find two days where the temperatures add up to a specific target value.

🎯 Goal: Build a program that uses the two pointer technique to find if there are two numbers in the array that add up to the target temperature.

📋 What You'll Learn

Create an integer array called temps with these exact values: 2, 4, 7, 11, 15, 20

Create an integer variable called target and set it to 18

Use two integer pointers left and right to scan the array from both ends

Print "Found pair: X and Y" if two numbers add up to target, else print "No pair found"

💡 Why This Matters

🌍 Real World

Finding pairs of values that meet a condition is common in data analysis, such as matching budgets, temperatures, or scores.

💼 Career

Two pointer technique is a fundamental skill for coding interviews and efficient array processing in software development.

1

Create the temperature array

Create an integer array called temps with these exact values: 2, 4, 7, 11, 15, 20

DSA C

// Create the integer array temps with values 2, 4, 7, 11, 15, 20
// Your code here

Hint

Use int temps[] = {2, 4, 7, 11, 15, 20}; to create the array.

2

Set the target sum

Create an integer variable called target and set it to 18

DSA C

#include 

int main() {
    int temps[] = {2, 4, 7, 11, 15, 20};
    // Create the integer variable target and set it to 18
    // Your code here
    return 0;
}

Hint

Use int target = 18; to create the target variable.

3

Implement the two pointer search

Create two integer variables called left and right. Set left to 0 and right to 5. Use a while loop with condition left < right. Inside the loop, calculate the sum of temps[left] and temps[right]. If the sum equals target, break the loop. If the sum is less than target, increment left. Otherwise, decrement right.

DSA C

#include 

int main() {
    int temps[] = {2, 4, 7, 11, 15, 20};
    int target = 18;
    // Initialize left and right pointers
    // Use a while loop to find two numbers that add to target
    // Your code here
    return 0;
}

Hint

Use two pointers starting at the ends of the array and move them based on the sum compared to target.

4

Print the result

After the loop, use an if statement to check if the variable found is 1. If yes, print "Found pair: X and Y" where X and Y are temps[left] and temps[right]. Otherwise, print "No pair found".

DSA C

Hint

Use printf to display the found pair or no pair message.

Practice

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

easy

A. Use one pointer to traverse and another to store the sum.

B. Use two arrays to store elements separately.

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

D. Use two pointers to swap elements randomly.

Solution

1. Step 1: Understand the technique

   The two pointer technique involves using two indices or pointers to scan the array, often from the start and end, to solve problems efficiently.

2. Step 2: Identify the correct description

   Use two indices to traverse the array from different ends or positions simultaneously. correctly describes using two pointers to traverse the array from different ends or positions simultaneously.

3. Final Answer:

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

4. Quick Check:

   Two pointers move from different positions = A [OK]

Hint: Two pointers move from different ends or positions [OK]

Common Mistakes:

• Thinking two pointers mean two separate arrays
• Confusing pointers with storing sums
• Assuming pointers swap elements randomly

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

easy

A. int left = 0, right = n - 1;

B. int left = 1, right = n;

C. int left = n, right = 0;

D. int left = n - 1, right = 0;

Solution

1. Step 1: Understand pointer initialization

   For two pointers starting at the beginning and end of an array, left should be 0 and right should be n - 1 because array indices start at 0.

2. Step 2: Check options

   int left = 0, right = n - 1; correctly initializes left = 0 and right = n - 1. Other options have incorrect indices.

3. Final Answer:

   int left = 0, right = n - 1; -> Option A

4. Quick Check:

   Start at 0 and end at n-1 = C [OK]

Hint: Array indices start at 0, so right = n - 1 [OK]

Common Mistakes:

• Starting left at 1 instead of 0
• Setting right to n (out of bounds)
• Swapping left and right initialization

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

int arr[] = {1, 3, 5, 7, 9};
int left = 0, right = 4;
int sum = 0;
while (left < right) {
    sum += arr[left] + arr[right];
    left++;
    right--;
}
printf("%d", sum);

medium

A. 25

B. 15

C. 30

D. 20

Solution

1. Step 1: Trace the loop iterations

   Initial pointers: left=0, right=4
   Iteration 1: sum=0+(1+9)=10, left=1, right=3
   Iteration 2: sum=10+(3+7)=20, left=2, right=2
   Loop ends because left is not less than right.

2. Step 2: Calculate final sum

   Sum after loop = 20. Note that the middle element (index 2) is not added because left == right stops the loop.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   Sum pairs (1+9)+(3+7) = 20 [OK]

Hint: Sum pairs while left < right, middle element skipped [OK]

Common Mistakes:

• Including middle element when left == right
• Adding elements twice
• Incorrect loop condition

4. Identify the error in the following code snippet that uses the two pointer technique to find if a sorted array contains two numbers that sum to a target value.

int arr[] = {2, 4, 6, 8, 10};
int left = 0, right = 4;
int target = 14;
while (left <= right) {
    int sum = arr[left] + arr[right];
    if (sum == target) {
        printf("Found");
        break;
    } else if (sum < target) {
        left++;
    } else {
        right--;
    }
}

medium

A. The pointers left and right are initialized incorrectly.

B. The loop condition should be left < right, not left <= right.

C. The sum comparison logic is reversed.

D. The printf statement should be outside the loop.

Solution

1. Step 1: Analyze loop condition

   The loop uses left <= right, which allows pointers to meet or cross. This can cause incorrect checks or double counting.

2. Step 2: Correct loop condition

   Using left < right ensures pointers do not cross and only valid pairs are checked.

3. Final Answer:

   The loop condition should be left < right, not left <= right. -> Option B

4. Quick Check:

   Pointers must not cross: left < right [OK]

Hint: Use left < right to avoid pointer crossing [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Swapping pointer increment/decrement logic
• Incorrect pointer initialization

5. Given a sorted array arr of size n, how can you use the two pointer technique to find all unique pairs that sum to a target value t? Choose the best approach.

hard

A. Initialize two pointers at start and end; move pointers inward based on sum comparison; skip duplicates to find unique pairs.

B. Use nested loops to check all pairs and store sums in a hash set.

C. Sort the array and use binary search for each element to find complement.

D. Initialize both pointers at start and move both forward to find pairs.

Solution

1. Step 1: Use two pointers at array ends

   Start with left = 0 and right = n - 1. Calculate sum of elements at these pointers.

2. Step 2: Move pointers and skip duplicates

   If sum equals target, record pair and move both pointers skipping duplicates. If sum < target, move left pointer right. If sum > target, move right pointer left.

3. Final Answer:

   Initialize two pointers at start and end; move pointers inward based on sum comparison; skip duplicates to find unique pairs. -> Option A

4. Quick Check:

   Two pointers from ends + skip duplicates = D [OK]

Hint: Move pointers inward and skip duplicates for unique pairs [OK]

Common Mistakes:

• Using nested loops instead of two pointers
• Not skipping duplicates causing repeated pairs
• Moving both pointers forward instead of inward