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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

Author: 3 people

backtracking/crossword_puzzle_solver.py

@@ -0,0 +1,132 @@
+# https://www.geeksforgeeks.org/solve-crossword-puzzle/
+
+
+def is_valid(
+    puzzle: list[list[str]], word: str, row: int, col: int, vertical: bool
+) -> bool:
+    """
+    Check if a word can be placed at the given position.
+
+    >>> puzzle = [
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', '']
+    ... ]
+    >>> is_valid(puzzle, 'word', 0, 0, True)
+    True
+    >>> puzzle = [
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', '']
+    ... ]
+    >>> is_valid(puzzle, 'word', 0, 0, False)
+    True
+    """
+    for i in range(len(word)):
+        if vertical:
+            if row + i >= len(puzzle) or puzzle[row + i][col] != "":
+                return False
+        else:
+            if col + i >= len(puzzle[0]) or puzzle[row][col + i] != "":
+                return False
+    return True
+
+
+def place_word(
+    puzzle: list[list[str]], word: str, row: int, col: int, vertical: bool
+) -> None:
+    """
+    Place a word at the given position.
+
+    >>> puzzle = [
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', '']
+    ... ]
+    >>> place_word(puzzle, 'word', 0, 0, True)
+    >>> puzzle
+    [['w', '', '', ''], ['o', '', '', ''], ['r', '', '', ''], ['d', '', '', '']]
+    """
+    for i, char in enumerate(word):
+        if vertical:
+            puzzle[row + i][col] = char
+        else:
+            puzzle[row][col + i] = char
+
+
+def remove_word(
+    puzzle: list[list[str]], word: str, row: int, col: int, vertical: bool
+) -> None:
+    """
+    Remove a word from the given position.
+
+    >>> puzzle = [
+    ...     ['w', '', '', ''],
+    ...     ['o', '', '', ''],
+    ...     ['r', '', '', ''],
+    ...     ['d', '', '', '']
+    ... ]
+    >>> remove_word(puzzle, 'word', 0, 0, True)
+    >>> puzzle
+    [['', '', '', ''], ['', '', '', ''], ['', '', '', ''], ['', '', '', '']]
+    """
+    for i in range(len(word)):
+        if vertical:
+            puzzle[row + i][col] = ""
+        else:
+            puzzle[row][col + i] = ""
+
+
+def solve_crossword(puzzle: list[list[str]], words: list[str]) -> bool:
+    """
+    Solve the crossword puzzle using backtracking.
+
+    >>> puzzle = [
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', '']
+    ... ]
+
+    >>> words = ['word', 'four', 'more', 'last']
+    >>> solve_crossword(puzzle, words)
+    True
+    >>> puzzle = [
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', ''],
+    ...     ['', '', '', '']
+    ... ]
+    >>> words = ['word', 'four', 'more', 'paragraphs']
+    >>> solve_crossword(puzzle, words)
+    False
+    """
+    for row in range(len(puzzle)):
+        for col in range(len(puzzle[0])):
+            if puzzle[row][col] == "":
+                for word in words:
+                    for vertical in [True, False]:
+                        if is_valid(puzzle, word, row, col, vertical):
+                            place_word(puzzle, word, row, col, vertical)
+                            words.remove(word)
+                            if solve_crossword(puzzle, words):
+                                return True
+                            words.append(word)
+                            remove_word(puzzle, word, row, col, vertical)
+                return False
+    return True
+
+
+if __name__ == "__main__":
+    PUZZLE = [[""] * 3 for _ in range(3)]
+    WORDS = ["cat", "dog", "car"]
+
+    if solve_crossword(PUZZLE, WORDS):
+        print("Solution found:")
+        for row in PUZZLE:
+            print(" ".join(row))
+    else:
+        print("No solution found:")