🏆 Classical DSA Problems

Timeless DSA problems that appear in every coding interview. Each with detailed explanations, multiple approaches, PHP implementations, and complexity analysis.

📝 Tower of Hanoi

Medium

Recursive puzzle: move disks between pegs.

📝 N Queens

Hard

Backtracking: place N queens on chessboard.

📝 Sudoku Solver

Hard

Backtracking: fill 9×9 Sudoku grid.

📝 Rat in a Maze

Medium

Backtracking: find path through maze.

📝 Traveling Salesman Problem

Hard

NP-Hard: shortest route visiting all cities.

📝 Job Scheduling

Medium

Greedy: schedule jobs for maximum profit.

📝 Activity Selection

Easy

Greedy: select maximum non-overlapping activities.

📝 Huffman Coding

Medium

Greedy: optimal prefix codes for compression.

📝 Number of Islands

Medium

Graph/DFS: count islands in 2D grid.

📝 Kth Largest Element

Easy

Heap/QuickSelect: find Kth largest element.

📝 Trapping Rain Water

Medium

Array/Two-pointer: trapped water between bars.

📝 Merge Intervals

Easy

Array: merge overlapping intervals.

📝 Two Sum

Easy

Hash Map: find pair summing to target.

📝 Three Sum

Medium

Two Pointer: find triplets summing to zero.

📝 Maximum Subarray

Easy

Kadane: max sum contiguous subarray.