Solution.md

Elevator

The goal of this contract is to reach the top floor of the building or set top equal to true.

The gotTo() function is the only function we can call in Elevator:

function goTo(uint _floor) public {
    Building building = Building(msg.sender);

    if (! building.isLastFloor(_floor)) {
      floor = _floor;
      top = building.isLastFloor(floor);
    }
}

The function creates an instance of the Building interface at the address of msg.sender. The Building interface is as follows:

interface Building {
  function isLastFloor(uint) external returns (bool);
}

Building has a function isLastFloor(), which returns a boolean. In order to set top = true in goTo(), building.isLastFloor() must return false to pass the if condition, and then return true in order to set the top. There are two ways to go about this but both require some sort of counter variable to keep track of:

• Have a boolean that we set conditionally
• Have a uint256 that we iterate and perform checks based on its value

The isLastFloor() function in AttackElevator.sol uses a boolean to pass these conditions. To pass the level, simply call the hack() function. hack() sends the elevator to the first floor, which is the returned as the last floor according to our isLastFloor() function.