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Clarify the problem constraints (e.g., adjacency definition, revisiting cells, word length) and then propose a depth-first search (DFS) with backtracking from each cell that matches the first character. Explain how to mark visited cells and prune branches to achieve O(N * 3^L) time, where N is the number of cells and L is the word length.
Pro tip: Mention that you can optimize by checking character frequency counts upfront: if the grid lacks enough of any character in the word, return false immediately. Also, start DFS from the less frequent end of the word to reduce search space.
Ask about grid size, word length, allowed moves (4-directional vs 8), and whether cells can be reused. Discuss edge cases like empty grid, empty word, or word longer than total cells.
Propose DFS with backtracking as the standard approach. Explain that BFS is less suitable because we need to explore paths, not just reachability.
Define a recursive function that takes current position and index in word. If index equals word length, return true. If out of bounds, character mismatch, or cell already visited, return false. Otherwise, mark cell visited, recurse in four directions, then unmark.
Pre-check character counts to fail fast. Optionally, start from the end of the word if its last character is less frequent in the grid. Use in-place marking (e.g., replace with '#') to save space.
State time complexity O(N * 3^L) and space O(L) for recursion. Walk through a small example and discuss potential pitfalls like stack overflow for large grids.
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