Notes

DSA loops questions for interviews

5 min read

Haan bhai, loops ke questions interview mein kaafi common hote hain, kyunki isse tumhari logic building aur time complexity ka samajh test karte hain. Chal, kuch popular loop-based questions aur unke approaches dekhte hain:

1. Print Patterns

Question:

Print this pattern:

*
**
***
****
*****

Approach:

  • Use nested loops: Outer loop controls rows, inner loop controls columns.
for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        System.out.print("*");
    }
    System.out.println();
}

Output:
O(n²) (Outer loop: O(n), Inner loop: O(n) for each row).

2. Reverse a Number

Question:

Reverse the digits of a number (e.g., 12345 → 54321).

Approach:

  • Use a while loop to extract digits and reverse them.
int num = 12345, reversed = 0;
while (num != 0) {
    int digit = num % 10;
    reversed = reversed * 10 + digit;
    num /= 10;
}
System.out.println(reversed);

Time Complexity: O(d) (where d = number of digits).

3. Check Prime Number

Question:

Write a program to check if a number is prime.

Approach:

  • Use a loop to check divisors up to n\sqrt{n}.
boolean isPrime = true;
int n = 29;
for (int i = 2; i <= Math.sqrt(n); i++) {
    if (n % i == 0) {
        isPrime = false;
        break;
    }
}
System.out.println(isPrime ? "Prime" : "Not Prime");

Time Complexity: O(√n).

4. Find the Largest and Smallest in an Array

Question:

Find the largest and smallest numbers in an array.

Approach:

  • Use a single loop to compare each element.
int[] arr = {3, 5, 7, 2, 8};
int max = arr[0], min = arr[0];
for (int num : arr) {
    if (num > max) max = num;
    if (num < min) min = num;
}
System.out.println("Max: " + max + ", Min: " + min);

Time Complexity: O(n).

5. Count Frequency of Each Element in an Array

Question:

Count the frequency of each element in an array.

Approach:

  • Use nested loops or a HashMap for efficient counting.
int[] arr = {1, 2, 2, 3, 3, 3};
Map<Integer, Integer> frequency = new HashMap<>();
for (int num : arr) {
    frequency.put(num, frequency.getOrDefault(num, 0) + 1);
}
System.out.println(frequency);

Time Complexity: O(n) (using HashMap).

6. Find Sum of Digits of a Number

Question:

Find the sum of digits in a number (e.g., 123 → 1 + 2 + 3 = 6).

Approach:

  • Use a while loop to extract digits and sum them.
int num = 123, sum = 0;
while (num != 0) {
    sum += num % 10;
    num /= 10;
}
System.out.println(sum);

Time Complexity: O(d) (where d = number of digits).

7. Find Factorial of a Number

Question:

Find the factorial of a number (e.g., 5! = 5 × 4 × 3 × 2 × 1 = 120).

Approach:

  • Use a for loop for multiplication.
int n = 5, fact = 1;
for (int i = 1; i <= n; i++) {
    fact *= i;
}
System.out.println(fact);

Time Complexity: O(n).

8. Fibonacci Series

Question:

Generate the first n Fibonacci numbers.

Approach:

  • Use a for loop to calculate Fibonacci iteratively.
int n = 10, a = 0, b = 1;
System.out.print(a + " " + b + " ");
for (int i = 2; i < n; i++) {
    int next = a + b;
    System.out.print(next + " ");
    a = b;
    b = next;
}

Time Complexity: O(n).

9. Print All Pairs in an Array

Question:

Print all pairs of numbers from an array.

Approach:

  • Use nested loops to generate pairs.
int[] arr = {1, 2, 3};
for (int i = 0; i < arr.length; i++) {
    for (int j = i + 1; j < arr.length; j++) {
        System.out.println(arr[i] + ", " + arr[j]);
    }
}

Time Complexity: O(n²).

10. Check for Palindrome String

Question:

Check if a string is a palindrome (e.g., madam).

Approach:

  • Use a for loop to compare characters from both ends.
String str = "madam";
boolean isPalindrome = true;
for (int i = 0; i < str.length() / 2; i++) {
    if (str.charAt(i) != str.charAt(str.length() - 1 - i)) {
        isPalindrome = false;
        break;
    }
}
System.out.println(isPalindrome ? "Palindrome" : "Not Palindrome");

Time Complexity: O(n).

11. Count Number of Words in a Sentence

Question:

Count the number of words in a sentence.

Approach:

  • Use a loop to split and count words.
String sentence = "This is a test sentence";
String[] words = sentence.split(" ");
System.out.println("Word count: " + words.length);

Time Complexity: O(n).

12. Find GCD of Two Numbers

Question:

Find the greatest common divisor (GCD) of two numbers.

Approach:

  • Use a while loop with the Euclidean algorithm.
int a = 56, b = 98;
while (b != 0) {
    int temp = b;
    b = a % b;
    a = temp;
}
System.out.println("GCD: " + a);

Time Complexity: O(log(min(a, b))).

Bhai, inn questions ko achhi tarah practice kar lo. Interview mein loops ke alag-alag variations pooch sakte hain. Agar kisi question ka doubt ho, toh turant pooch lena! 🚀

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