a.yaml

openapi: 3.0.0
info:
  title: ""
  version: ""
servers:
  - url: /
paths:
  /api/foo:
    get:
      requestBody:
        content:
          application/json;charset=utf-8:
            schema:
              $ref: "#/components/schemas/Tree"

      responses:
        200:
          description: ""
          content:
            application/json;charset=utf-8:
              schema:
                $ref: "#/components/schemas/Tree"
components:
  schemas:
    Tree:
      type: object
      properties:
        node:
          required:
            - children
          type: object
          properties:
            children:
              type: array
              items:
                $ref: "#/components/schemas/Tree"
        leaf:
          required:
            - value
          type: object
          properties:
            value:
              $ref: "#/components/schemas/Item"
    Item:
      required:
        - foo
      type: object
      properties:
        foo:
          type: string