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Find if a word exists in a grid.
Problem: Given a grid and word, check if word exists as a path of adjacent cells (no reuse).
Approach: DFS from each cell matching the first character. Mark cells visited by modifying temporarily, restore after. Explore all four directions. Return true if any path completes the word.
Time: where is word length.
Space: for recursion stack.
See the implementation unit for code.
