Most Commonly Asked DSA Interview Questions

Q: How do you reverse a linked list?

• Answer: Reversing a linked list involves changing the direction of its pointers so that the list starts from the last element and ends at the first.
• Example: Input: 1 -> 2 -> 3 -> 4 -> null Output: 4 -> 3 -> 2 -> 1 -> null

Q: How do you perform binary search on a sorted array?

• Answer: Binary search divides the array in half repeatedly, checking if the middle element matches the target.
• Example: Input: Array [1, 3, 5, 7, 9], Target = 7 Output: 3 (index of 7)
• Solution Approach: Check the middle element; if it’s the target, return the index. If the target is smaller, search the left half; if it is larger, search the right half.

Q: How do you find the first unique character in a string?

• Answer: To find the first unique character, count each character’s occurrences and identify the first one that appears only once.
• Example: Input: "Swiss" Output: "w"
• Solution Approach: Use a hash map to store each character’s frequency, then iterate through the string to find the first character with a count of 1.

Q: How do you detect a cycle in a linked list?

• Answer: To detect a cycle in a linked list, use two pointers (slow and fast). If there’s a cycle, the fast pointer will eventually meet the slow pointer.
• Example: Input: 1 -> 2 -> 3 -> 4 -> 2 (cycle) Output: True (cycle exists)
• Approach: Use Floyd’s Cycle Detection algorithm. Move the fast pointer two steps and the slow pointer one step. If they meet, there’s a cycle.

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