python - python 中奇怪的递归行为

Question

我的数独解算器完全按照它应该做的去做 - 除了返回正确的东西。它在返回(正确求解的网格)之前打印它应该打印的内容，但随后它似乎继续运行了一段时间并返回 None。我不知道发生了什么。

网格是列表的列表。假设如果网格有效(已解决或未解决)，check_sudoku 返回 True，否则返回 False。

def solve_sudoku(grid, row=0, col=0):
    "Searches for valid numbers to put in blank cells until puzzle is solved."
    # Sanity check
    if not check_sudoku(grid): 
        return None
    # Sudoku is solved
    if row > 8: 
        return grid
    # not a blank, try next cell
    elif grid[row][col] != 0:
        next_cell(grid, row, col)
    else:
        # try every number from 1-9 for current cell until one is valid
        for n in range(1, 10):
            grid[row][col] = n
            if check_sudoku(grid):
                next_cell(grid, row, col)
        else:
            # Sudoku is unsolvable at this point, clear cell and backtrack
            grid[row][col] = 0
            return

def next_cell(grid, row, col):
    "Increments column if column is < 8 otherwise increments row"
    return solve_sudoku(grid, row, col+1) if col < 8 else solve_sudoku(grid, row+1, 0)

Answer 1

您在递归中调用 next_cell，但从未返回其值。