A simple FastAPI endpoint takes a binary tree and reverses it.

fastapi_reverse_tree.py

from fastapi import FastAPI
from typing import List, Optional
from pydantic import BaseModel

app = FastAPI()


class Node(BaseModel):
    value: Optional[int]
    left: Optional["Node"] = None
    right: Optional["Node"] = None


def reverse_tree(root):
    if root is None:
        return None
    else:
        temp = root.left
        root.left = reverse_tree(root.right)
        root.right = reverse_tree(temp)
        return root


@app.post("/reverse_tree", response_model=Node)
async def reverse_binary_tree(node: Node) -> Node:
    """
    Reverses a binary tree.

    Example usage:
    ```
    {
      "value": 4,
      "left": {
        "value": 2,
        "left": {
          "value": 1
        },
        "right": {
          "value": 3
        }
      },
      "right": {
        "value": 7,
        "left": {
          "value": 6
        },
        "right": {
          "value": 9
        }
      }
    }
    ```
    """
    reversed_root = reverse_tree(node)
    return reversed_root