DSA interview questions and answers

Use a dummy head and append the smaller current node from either list, then attach any leftover tail.

Walk both lists with a carry, creating a new node for (sum % 10) and carrying sum // 10.

Find the middle with slow/fast, reverse the second half, compare pairwise, then optionally restore.

Two pointers walk both lists and switch to the other head on null

Split at the middle, reverse the second half, then weave nodes from the two halves.

The diameter is the max of left_height + right_height over every node (in edges).

Recurse: if the current node is p or q, return it

DFS subtracting the node value, succeeding when you hit a leaf and remaining target is zero.

A bottom-up DFS returns height or a sentinel if any subtree differs by more than one.

Level-order BFS and take the last node of each level, or DFS root-right-left and record the first node seen at each depth.

DFS with a legal (low, high) window: left child must be < node and stay > low, right must be > node and stay < high.

Inorder traversal visits keys in order

Walk from the root: go left if both keys are smaller, right if both are larger

Always pick the middle element as root and recurse on left and right halves so subtree sizes differ by at most one.

Count frequencies in a hash map, then push (freq, value) into a min-heap of size k.

Keep a max-heap of size k by squared distance so the farthest of the closest k sits at the top.

Count frequencies, reject if the max count exceeds (n+1)/2, then always place the current most frequent remaining character from a max-heap, holding the previous one out for a turn.

Push the first column (or first row) into a min-heap of (value, r, c) and pop k times, pushing the next item in that row each pop.

Scan cells

Hash original node to clone.