Two Pointer Technique on Arrays in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Two Pointer Sum Check

What is the output of the following C code that uses two pointers to check if any two numbers in a sorted array sum to a target value?

DSA C

#include <stdio.h>

int main() {
    int arr[] = {1, 3, 4, 5, 7, 10, 12};
    int target = 9;
    int left = 0, right = 6;
    int found = 0;

    while (left < right) {
        int sum = arr[left] + arr[right];
        if (sum == target) {
            found = 1;
            break;
        } else if (sum < target) {
            left++;
        } else {
            right--;
        }
    }

    if (found) {
        printf("Found: %d + %d = %d\n", arr[left], arr[right], target);
    } else {
        printf("Not Found\n");
    }
    return 0;
}

A Found: 4 + 5 = 9

B Found: 3 + 7 = 9

C Not Found

D Found: 1 + 12 = 9

💡 Hint

Think about how the two pointers move based on the sum compared to the target.

The two pointers start at the ends: 1 and 12 sum to 13 which is greater than 9, so right pointer moves left. Then 1 and 10 sum to 11, still greater, right moves left. Then 1 and 7 sum to 8, less than 9, left moves right. Then 3 and 7 sum to 10, greater, right moves left. Then 3 and 5 sum to 8, less, left moves right. Then 4 and 5 sum to 9, found the pair.

🧠 Conceptual

intermediate

Two Pointer Technique Purpose

What is the main advantage of using the two pointer technique on a sorted array compared to a nested loop approach?

A It uses less memory by storing fewer elements

B It reduces time complexity from O(n^2) to O(n)

C It sorts the array automatically

D It allows searching for multiple targets simultaneously

💡 Hint

Think about how many times each element is visited in two pointer vs nested loops.

The two pointer technique moves pointers inward in a single pass, checking pairs efficiently, reducing time complexity from quadratic to linear.

🔧 Debug

advanced

Identify the Bug in Two Pointer Implementation

What error will the following code produce when trying to find if two numbers sum to target in a sorted array?

DSA C

#include <stdio.h>

int main() {
    int arr[] = {2, 4, 6, 8, 10};
    int target = 14;
    int left = 0, right = 4;

    while (left <= right) {
        int sum = arr[left] + arr[right];
        if (sum == target) {
            printf("Found: %d + %d = %d\n", arr[left], arr[right], target);
            return 0;
        } else if (sum < target) {
            left++;
        } else {
            right--;
        }
    }
    printf("Not Found\n");
    return 0;
}

A Infinite loop because left can become equal to right and pointers do not move

B Runs correctly and prints Not Found

C Runs correctly and prints Found: 4 + 10 = 14

D Runs correctly but prints Found: 6 + 8 = 14

💡 Hint

Check the loop condition and pointer movements carefully.

The loop condition 'left <= right' is valid here because when left == right, sum is arr[left]*2 which won't equal target 14. The pointers move correctly and the pair 4 + 10 = 14 is found.

❓ Predict Output

advanced

Output of Two Pointer Removing Duplicates

What is the output of this C code that removes duplicates from a sorted array using two pointers?

DSA C

#include <stdio.h>

int main() {
    int arr[] = {1, 1, 2, 2, 3, 4, 4, 5};
    int n = 8;
    int i = 0, j = 1;

    while (j < n) {
        if (arr[i] != arr[j]) {
            i++;
            arr[i] = arr[j];
        }
        j++;
    }

    printf("Array after removing duplicates: ");
    for (int k = 0; k <= i; k++) {
        printf("%d ", arr[k]);
    }
    printf("\n");
    return 0;
}

A Array after removing duplicates: 1 2 2 3 4 5

B Array after removing duplicates: 1 1 2 3 4 5

C Array after removing duplicates: 1 2 3 4 4 5

D Array after removing duplicates: 1 2 3 4 5

💡 Hint

Check how the pointers move and how duplicates are skipped.

Pointer i tracks the last unique element index. Pointer j scans forward. When a new unique element is found, i increments and arr[i] is updated. Result is unique elements at start: 1 2 3 4 5.

🧠 Conceptual

expert

Two Pointer Technique Limitations

Which of the following is a limitation of the two pointer technique on arrays?

A It requires the array to be sorted or have a specific order

B It cannot be used to find pairs with a given sum

C It always uses extra memory proportional to array size

D It only works on arrays with even number of elements

💡 Hint

Think about the assumptions needed for two pointers to work correctly.

Two pointer technique relies on sorted or ordered arrays to move pointers logically. Without order, it cannot guarantee correct results efficiently.