def solve_nqueens(n):
    def is_safe(board, row, col):
        for i in range(row):
            if (board[i] == col or
                abs(board[i] - col) == abs(i - row)):
                return False
        return True

    def backtrack(board, row):
        if row == n:
            solutions.append(board[:])
            return
        for col in range(n):
            if is_safe(board, row, col):
                board[row] = col       # place
                backtrack(board, row + 1)
                board[row] = -1        # undo

    solutions = []
    backtrack([-1] * n, 0)
    return solutions

# 4-Queens: 0 solutions