dfs

Graph traversal sits beneath a large family of interview problems. What matters is knowing the BFS-vs-DFS tradeoff (shortest path vs memory shape) and applying it cleanly, then getting the directed-vs-undirected cycle gotcha that trips up most candidates.

Backtracking underlies permutations, subsets, N-queens, and constraint problems. Interviewers look for the choose/explore/un-choose template plus pruning, and knowing exactly which knob distinguishes permutations from combinations.

A staple grid-traversal problem probing BFS/DFS flood fill and disciplined visited-marking. What interviewers watch for: viewing the grid as a graph and tallying connected components. Here is the answer.

Turning a tree into a string and reconstructing it faithfully rests on one choice most candidates overlook. Skip it and the structure turns ambiguous. Here is the answer and the interview-grade reasoning.

LCA is the recursion staple that probes how you carry information upward from subtrees in one pass. What interviewers watch for: the post-order 'found in left, found in right' logic. Here is the answer plus the BST shortcut.

The diameter is not the tree's height, and the path does not have to pass through the root. The clean solution derives height and best path in one DFS, updating a global max at every node. Here is the answer and the subtlety candidates miss.

Preorder names the root; inorder divides the rest into left and right subtrees. The naive version is O(n squared); the strong answer uses a value-to-index map and a moving preorder pointer for O(n). Here is the answer and why both orders are required.

Negative subtrees, a path that bends through any node, and a return value distinct from the answer you track: this problem stuffs three traps into one DFS. Here is the clean O(n) solution and the reasoning that holds up under follow-ups.

Generating then filtering costs O(2^(2n)) and wastes effort. Backtracking with two counters builds only valid strings by applying the balance rule at every step. Here is the template and why the pruning conditions are exactly right.