Interview Q&A / Data Structures & Algorithms

1. How do you detect a cycle in a linked list?

   Use Floyd's tortoise and hare method. Move a slow pointer one step and a fast pointer two steps at a time. If the fast pointer reaches null, there is no cycle. If the two pointers ever meet, there is a cycle. It runs in O(n) time with O(1) extra space.

   IntermediateIn-depthCoding4 min read
2. How do you check if a linked list is a palindrome in O(1) extra space?

   Find the middle with slow and fast pointers, reverse the second half in place, then compare it with the first half node by node.

   IntermediateQuick answerCoding1 min read
3. How do you find all unique triplets in an array that sum to zero?

   Sort the array, fix one number, and use two pointers on the rest to find pairs that cancel it. Skip equal values to avoid duplicate triplets. It runs in O(n squared).

   IntermediateQuick answerCoding1 min read
4. How do you find the longest palindromic substring in a string?

   Every palindrome has a centre. Try each index as an odd-length centre and each gap between letters as an even-length centre, expand outward while the ends match, and keep the longest.

   IntermediateQuick answerCoding1 min read
5. How do you find the node where two linked lists intersect?

   Walk two pointers, one from each head. When a pointer reaches the end, redirect it to the head of the other list. After at most two passes they meet at the intersection node, or both become null if there is none.

   IntermediateQuick answerCoding1 min read
6. How do you remove the nth node from the end of a linked list in one pass?

   Move a fast pointer n steps ahead of a slow pointer, then move both until fast reaches the last node. The slow pointer is then just before the node to delete. A dummy head handles removing the first node.

   IntermediateQuick answerCoding1 min read
7. How do you rotate an array to the right by k steps in place?

   Reverse the whole array, then reverse the first k elements, then reverse the rest. Take k modulo the length first. It runs in O(n) time with O(1) extra space.

   IntermediateQuick answerCoding1 min read
8. How do you solve the Container With Most Water problem using two pointers?

   Start with pointers at both ends. The area is the shorter height times the distance. Move the pointer at the shorter line inward, because moving the taller one can never increase the area.

   IntermediateQuick answerCoding1 min read
9. How do you sort an array of 0s, 1s and 2s in a single pass?

   Use the Dutch national flag method with three pointers: low for the next 0 position, high for the next 2 position, and mid scanning. Swap 0s to the front and 2s to the back; leave 1s in the middle.

   IntermediateQuick answerCoding1 min read