Classic backtracking DFS, mark cells visited as you go (or swap in a sentinel character to avoid an extra array).
This is a classic backtracking problem on a grid. Start by iterating over each cell as a potential starting point, then perform DFS to match the target word character by character, marking visited cells and backtracking when necessary. Optimize with early termination if the first character doesn't match or if the word is longer than the total cells.
Pro tip: Mention that you can optimize by checking character frequency counts upfront: if the word requires more of a character than present in the grid, return false immediately. Also, discuss pruning by starting DFS only from cells that match the first character.
Confirm edge cases: empty grid, empty word, word longer than grid cells, and character set constraints. Ask if diagonal moves are allowed (they are not).
Explain that you'll use DFS to explore all four directions from each cell, marking cells as visited to avoid reuse, and unmarking them when backtracking.
Write a recursive function that takes the current cell coordinates and the index in the target word. Base case: if index equals word length, return true. Check bounds, visited status, and character match before recursing.
Loop through each cell in the grid; if the cell matches the first character of the word, start DFS from there. Return true if any DFS succeeds.
State time complexity O(m*n*4^L) where L is word length, and space O(L) for recursion. Mention optimizations like frequency check and early pruning.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
First, restate the original problem to confirm understanding, then contrast it with the new constraint. Explain how the search space and algorithm change: from exploring all paths (DFS/backtracking) to checking only four straight lines per starting cell, and discuss the resulting time complexity improvement.
Pro tip: Mention that the straight-line constraint allows precomputing all possible lines (rows, columns, diagonals if allowed) and using string matching algorithms like KMP or rolling hash for efficient search, showing you think beyond brute force.
Briefly restate the original word search problem (e.g., Boggle-style) to ensure alignment and highlight the contrast with the new constraint.
Explain that words must be in a straight line in one of four cardinal directions, starting from any cell, with consecutive letters along that line.
Discuss how the search space reduces from exponential paths to linear checks per direction, and how this simplifies the algorithm (e.g., no backtracking needed).
Outline an approach: for each cell and each direction, check if the word matches the sequence of letters along that line, possibly using early termination or string matching optimizations.
Mention time/space complexity improvements, handling of boundaries, and whether diagonals or other directions are allowed.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.